diff --git a/components/ILIAS/Language/README.md b/components/ILIAS/Language/README.md
index fdf90ff455a7..4ff6fbb8458f 100755
--- a/components/ILIAS/Language/README.md
+++ b/components/ILIAS/Language/README.md
@@ -172,6 +172,12 @@ Only other modules (plugins) are read from `lng_data`/`lng_modules`.
 * Without a build, for a language the build lacks, or for a language that is
   not installed a shipped module has no texts (`-identifier-`); a missing
   build or a shipped `.po` that is not built is reported.
+* A real locale in `LC_MESSAGES` (e.g. from the setting "Locale" of the
+  general settings) stays, native gettext sets one only where `LC_MESSAGES`
+  is `C`, `POSIX` or `C.<codeset>`. ICU takes its default locale from
+  `LC_MESSAGES` once per process: a PHP-FPM worker whose first ICU call comes
+  before that setting or without it keeps `ilias__MESSAGES` (glibc) for its
+  lifetime.
 * Requirements: the PHP extension `gettext` for CLI and web server, and
   * glibc: the locale `C.utf8`, with the same C library for both; native
     gettext activates the build's copy of it through `LOCPATH`;
@@ -180,7 +186,8 @@ Only other modules (plugins) are read from `lng_data`/`lng_modules`.
     the libc there ignores `LOCPATH`, no locale is shipped (`C.UTF-8` does not
     work: GNU libintl ignores `LANGUAGE` there); locale variables of the
     environment (e.g. `LANG=C.UTF-8`) do not matter: native gettext sets the
-    environment variable `LC_ALL` to that locale (child processes inherit it);
+    environment variable `LC_ALL` to the active locale, a kept one too (child
+    processes inherit it);
   * musl is not supported (ignores `LOCPATH`).
 
   Build on the platform that serves ILIAS (with Docker in the container): the
diff --git a/components/ILIAS/Language/src/ComponentTranslation/NativeGettext.php b/components/ILIAS/Language/src/ComponentTranslation/NativeGettext.php
index 4dadb768d912..3372aae7506e 100644
--- a/components/ILIAS/Language/src/ComponentTranslation/NativeGettext.php
+++ b/components/ILIAS/Language/src/ComponentTranslation/NativeGettext.php
@@ -25,7 +25,8 @@
  * migrated modules, and the one place that calls it.
  *
  * Native gettext only translates while LC_MESSAGES is not "C"/"POSIX" and finds a catalog as
- * `<bound directory>/<language>/LC_MESSAGES/<domain>.mo`. ILIAS does not depend on the locale's language:
+ * `<bound directory>/<language>/LC_MESSAGES/<domain>.mo`. ILIAS does not depend on the locale's language, so a
+ * real locale in LC_MESSAGES (e.g. of ilInitialisation::initLocale()) stays. Only for "C"/"POSIX"/"C.<codeset>":
  * - glibc: the build ships its own locale (a copy of "C.utf8", see ShippedTranslationsBuild) named
  *   LOCALE_NAME; activate() loads it via LOCPATH and removes LOCPATH again right away (while it is set,
  *   setlocale() finds no system locale);
@@ -42,7 +43,8 @@
  * from translating). In a thread-safe (ZTS) PHP the end of another thread's request can take them back
  * mid-request: isActive() checks, reactivateIfLost() activates again (the administration shows a hint,
  * see isThreadSafe()). Child processes inherit LANGUAGE=messages, which is harmless, and on macOS/BSD
- * LC_ALL=<active locale>, which overrides the locale of the environment for them.
+ * LC_ALL=<active locale> (a kept one too, e.g. the locale of the administration), which overrides the
+ * locale of the environment for them.
  *
  * The C library never reloads a catalog file it has read once (also not a replaced one), so every new
  * state (a build, an overlay revision) gets a directory of its own, and binding the domain loads it.
@@ -85,6 +87,8 @@ final class NativeGettext
 
     private static ?string $problem = null;
 
+    private static bool $messages_set = false;
+
     public static function isExtensionLoaded(): bool
     {
         return extension_loaded('gettext');
@@ -126,8 +130,8 @@ public static function useSystemLocaleForTests(?bool $system_locale): void
     }
 
     /**
-     * Activates the locale LOCALE_NAME of $locale_directory (or, see usesSystemLocale(), a system locale)
-     * for LC_MESSAGES and sets LANGUAGE; if it fails, getProblem() tells why.
+     * Sets LANGUAGE and, if LC_MESSAGES holds no real locale, the locale LOCALE_NAME of $locale_directory (or, see
+     * usesSystemLocale(), a system locale) for it; if it fails, getProblem() tells why.
      */
     public static function activate(string $locale_directory): bool
     {
@@ -141,12 +145,17 @@ public static function activate(string $locale_directory): bool
         if (self::usesSystemLocale()) {
             return self::activateSystemLocale($locale_directory);
         }
+        // also checked when a real locale stays: a build without its locale is broken
         if (!is_dir($locale_directory . '/' . self::LOCALE_NAME)) {
             self::$problem = sprintf('the locale "%s" is missing in "%s"', self::LOCALE_NAME, $locale_directory);
             return false;
         }
 
         self::$previous ??= self::processState();
+        $kept = self::keptLocale();
+        if ($kept !== null) {
+            return self::activated($locale_directory, $kept, false);
+        }
         $locpath = getenv('LOCPATH');
         putenv('LOCPATH=' . $locale_directory);
         $locale = setlocale(LC_MESSAGES, self::LOCALE_NAME);
@@ -157,13 +166,14 @@ public static function activate(string $locale_directory): bool
             return false;
         }
 
-        return self::activated($locale_directory, $locale);
+        return self::activated($locale_directory, $locale, true);
     }
 
     private static function activateSystemLocale(string $locale_directory): bool
     {
         self::$previous ??= self::processState();
-        $locale = setlocale(LC_MESSAGES, self::SYSTEM_LOCALES);
+        $kept = self::keptLocale();
+        $locale = $kept ?? setlocale(LC_MESSAGES, self::SYSTEM_LOCALES);
         if ($locale === false) {
             self::$problem = sprintf(
                 'none of the system locales %s exists for LC_MESSAGES (needed on %s, where native gettext cannot use the locale of the build)',
@@ -175,7 +185,25 @@ private static function activateSystemLocale(string $locale_directory): bool
         // GNU libintl on macOS reads the locale of the environment before setlocale()'s, and "C" there ignores LANGUAGE
         putenv('LC_ALL=' . $locale);
 
-        return self::activated($locale_directory, $locale);
+        return self::activated($locale_directory, $locale, $kept === null);
+    }
+
+    /** A real LC_MESSAGES locale stays, so ICU & Co. keep seeing it (e.g. the one of ilInitialisation::initLocale()). */
+    private static function keptLocale(): ?string
+    {
+        $locale = setlocale(LC_MESSAGES, '0');
+        if (
+            $locale === false
+            || $locale === 'C'
+            || $locale === 'POSIX'
+            || str_starts_with($locale, 'C.')
+            || $locale === self::LOCALE_NAME
+            || (self::$messages_set && $locale === self::$active_locale)
+        ) {
+            return null;
+        }
+
+        return $locale;
     }
 
     /**
@@ -186,11 +214,12 @@ private static function processState(): array
         return ['language' => getenv('LANGUAGE'), 'messages' => setlocale(LC_MESSAGES, '0'), 'all' => getenv('LC_ALL')];
     }
 
-    private static function activated(string $locale_directory, string $locale): bool
+    private static function activated(string $locale_directory, string $locale, bool $messages_set): bool
     {
         putenv('LANGUAGE=' . self::CATALOG_LANGUAGE);
         self::$locale_directory = $locale_directory;
         self::$active_locale = $locale;
+        self::$messages_set = $messages_set;
         self::$problem = null;
 
         return true;
@@ -224,17 +253,17 @@ public static function reactivateIfLost(): bool
     }
 
     /**
-     * Takes LANGUAGE, LC_MESSAGES and LC_ALL back to the state before the first activate(); settings changed by
-     * other code since stay. Only for long-running processes (e.g. Setup's build after its check), a web
-     * request needs no call. No-op in a thread-safe PHP, where other threads may use them. The activated
-     * directory is kept, so reactivateIfLost() can activate it again.
+     * Takes LANGUAGE, LC_MESSAGES (if activate() set it) and LC_ALL back to the state before the first
+     * activate(); settings changed by other code since stay. Only for long-running processes (e.g. Setup's build
+     * after its check), a web request needs no call. No-op in a thread-safe PHP, where other threads may use them.
+     * The activated directory is kept, so reactivateIfLost() can activate it again.
      */
     public static function restore(): void
     {
         if (self::$previous === null || self::isThreadSafe()) {
             return;
         }
-        if (self::$previous['messages'] !== false && setlocale(LC_MESSAGES, '0') === self::$active_locale) {
+        if (self::$messages_set && self::$previous['messages'] !== false && setlocale(LC_MESSAGES, '0') === self::$active_locale) {
             setlocale(LC_MESSAGES, self::$previous['messages']);
         }
         if (getenv('LANGUAGE') === self::CATALOG_LANGUAGE) {
@@ -316,6 +345,7 @@ public static function resetForTests(): void
         self::$system_locale_for_tests = null;
         self::$locale_directory = null;
         self::$active_locale = null;
+        self::$messages_set = false;
         self::$previous = null;
         self::$problem = null;
     }
diff --git a/components/ILIAS/Language/src/ComponentTranslation/ShippedTranslationsBuild.php b/components/ILIAS/Language/src/ComponentTranslation/ShippedTranslationsBuild.php
index dbe6940e5f4a..bc3dbd3d6854 100644
--- a/components/ILIAS/Language/src/ComponentTranslation/ShippedTranslationsBuild.php
+++ b/components/ILIAS/Language/src/ComponentTranslation/ShippedTranslationsBuild.php
@@ -573,6 +573,23 @@ private function findCollisions(array $values_per_language): array
      * @throws RuntimeException if native gettext does not serve the expected value
      */
     private function assertNativeGettextServes(string $build_directory, array $sample): void
+    {
+        $messages = setlocale(LC_MESSAGES, '0');
+        // with "C" activate() loads the shipped locale instead of keeping a real one, so the check covers it
+        setlocale(LC_MESSAGES, 'C');
+        try {
+            $this->assertActivatedNativeGettextServes($build_directory, $sample);
+        } finally {
+            if ($messages !== false) {
+                setlocale(LC_MESSAGES, $messages);
+            }
+        }
+    }
+
+    /**
+     * @param array{module: string, lang: string, key: string, value: string} $sample
+     */
+    private function assertActivatedNativeGettextServes(string $build_directory, array $sample): void
     {
         if (!NativeGettext::activate(MigratedLanguageFilePaths::buildLocaleDirectory($build_directory))) {
             throw new RuntimeException(sprintf('Native gettext is not available: %s.', (string) NativeGettext::getProblem()));
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFilePathsTest.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFilePathsTest.php
index 4a112d20a33c..7d01b0e61773 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFilePathsTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFilePathsTest.php
@@ -33,6 +33,8 @@
  */
 class MigratedLanguageFilePathsTest extends TestCase
 {
+    use RestoresProcessState;
+
     private string $root;
 
     protected function setUp(): void
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncLockTestCase.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncLockTestCase.php
index 68679cb87a43..f72e8a6a8ef2 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncLockTestCase.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncLockTestCase.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -29,6 +29,8 @@
  */
 abstract class MigratedLanguageFileSyncLockTestCase extends TestCase
 {
+    use RestoresProcessState;
+
     protected const string MODULE = 'stest';
 
     protected string $client_data_dir;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncPluralOverlayTest.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncPluralOverlayTest.php
index 7767ad435f78..b9e9fe92326f 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncPluralOverlayTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncPluralOverlayTest.php
@@ -34,6 +34,8 @@
  */
 class MigratedLanguageFileSyncPluralOverlayTest extends TestCase
 {
+    use RestoresProcessState;
+
     private const string MODULE = 'ptest';
 
     private string $fixture_directory;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRemarksTest.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRemarksTest.php
index ffec8d2cdd6b..1678e1877ca5 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRemarksTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRemarksTest.php
@@ -34,6 +34,8 @@
  */
 class MigratedLanguageFileSyncRemarksTest extends TestCase
 {
+    use RestoresProcessState;
+
     private const string MODULE = 'rtest';
 
     private string $fixture_directory;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRevisionTest.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRevisionTest.php
index 357c6b21d236..e32668944c5b 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRevisionTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncRevisionTest.php
@@ -34,6 +34,8 @@
  */
 class MigratedLanguageFileSyncRevisionTest extends TestCase
 {
+    use \RestoresProcessState;
+
     private const string MODULE = 'rvs';
 
     private string $root;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncSymlinkTest.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncSymlinkTest.php
index ed608c058f8d..aed72fbe56a5 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncSymlinkTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncSymlinkTest.php
@@ -37,6 +37,8 @@
  */
 class MigratedLanguageFileSyncSymlinkTest extends TestCase
 {
+    use RestoresProcessState;
+
     private const string MODULE = 'stest';
 
     private string $fixture_directory;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncTestCase.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncTestCase.php
index cf4af282ec5c..a24683419191 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncTestCase.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedLanguageFileSyncTestCase.php
@@ -35,6 +35,8 @@
  */
 abstract class MigratedLanguageFileSyncTestCase extends TestCase
 {
+    use RestoresProcessState;
+
     protected const string MODULE = 'stest';
 
     protected string $fixture_directory;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/MigratedTranslationsTest.php b/components/ILIAS/Language/tests/ComponentTranslation/MigratedTranslationsTest.php
index 677b06aa899c..9987dd969ddd 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/MigratedTranslationsTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/MigratedTranslationsTest.php
@@ -33,6 +33,8 @@
  */
 class MigratedTranslationsTest extends TestCase
 {
+    use \RestoresProcessState;
+
     private string $root;
     private string $client_data_dir;
     private string $build_directory;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/NativeGettextTest.php b/components/ILIAS/Language/tests/ComponentTranslation/NativeGettextTest.php
index ecf5ab20a1c9..c7225c04c20d 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/NativeGettextTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/NativeGettextTest.php
@@ -24,6 +24,7 @@
 use ILIAS\Language\ComponentTranslation\Catalog\TranslationEntry;
 use MigratedPoFixture;
 use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\RunInSeparateProcess;
 use PHPUnit\Framework\TestCase;
 
 /**
@@ -35,26 +36,18 @@
  */
 class NativeGettextTest extends TestCase
 {
+    use \RestoresProcessState;
+
     private const string DOMAIN = 'ntg.de';
+    private const string REAL_LOCALE = 'de_DE.UTF-8';
 
     private string $root;
     private string $build_directory;
     private string $locale_directory;
 
-    /**
-     * @var array{language: string|false, locpath: string|false, all: string|false, messages: string|false}
-     */
-    private array $process_state;
-
     protected function setUp(): void
     {
         MigratedPoFixture::resetRuntime();
-        $this->process_state = [
-            'language' => getenv('LANGUAGE'),
-            'locpath' => getenv('LOCPATH'),
-            'all' => getenv('LC_ALL'),
-            'messages' => setlocale(LC_MESSAGES, '0'),
-        ];
         $this->root = sys_get_temp_dir() . '/ilias_native_gettext_' . bin2hex(random_bytes(6));
         mkdir($this->root, 0775, true);
 
@@ -76,13 +69,6 @@ protected function setUp(): void
     protected function tearDown(): void
     {
         NativeGettext::resetForTests();
-        // whatever a test did to the process, the next one starts as this one did
-        putenv($this->process_state['language'] === false ? 'LANGUAGE' : 'LANGUAGE=' . $this->process_state['language']);
-        putenv($this->process_state['locpath'] === false ? 'LOCPATH' : 'LOCPATH=' . $this->process_state['locpath']);
-        putenv($this->process_state['all'] === false ? 'LC_ALL' : 'LC_ALL=' . $this->process_state['all']);
-        if ($this->process_state['messages'] !== false) {
-            setlocale(LC_MESSAGES, $this->process_state['messages']);
-        }
         MigratedPoFixture::removeDirectory($this->root);
         MigratedPoFixture::resetRuntime();
     }
@@ -186,6 +172,7 @@ public static function processSettingsBeforeSystemLocaleActivation(): iterable
     {
         yield 'C locale' => ['C', 'de'];
         yield 'the system locale itself, LANGUAGE unset' => ['system', false];
+        yield 'a real locale, which stays' => [self::REAL_LOCALE, 'de'];
     }
 
     #[DataProvider('processSettingsBeforeSystemLocaleActivation')]
@@ -193,12 +180,19 @@ public function testSystemLocaleIsOnlyActiveAfterActivateAndRestoreGivesTheProce
     {
         $this->requireSystemLocale();
         NativeGettext::useSystemLocaleForTests(true);
+        $kept = $locale === self::REAL_LOCALE;
+        if ($kept) {
+            $this->requireRealLocale();
+        }
         $locale = $locale === 'system' ? (string) setlocale(LC_MESSAGES, NativeGettext::SYSTEM_LOCALES) : (string) setlocale(LC_MESSAGES, $locale);
         putenv($language === false ? 'LANGUAGE' : 'LANGUAGE=' . $language);
         $this->assertFalse(NativeGettext::isActive(), 'a locale set by someone else is not our activation');
 
         $this->activateSystemLocale();
         $this->assertTrue(NativeGettext::isActive());
+        if ($kept) {
+            $this->assertSame($locale, setlocale(LC_MESSAGES, '0'), 'a real locale is not replaced');
+        }
 
         NativeGettext::restore();
 
@@ -398,6 +392,182 @@ public function testReactivateIfLostRepairsWhatOtherCodeChanged(\Closure $change
         $this->assertSame('Hallo', NativeGettext::translate(self::DOMAIN, 'greeting'));
     }
 
+    private function requireRealLocale(): void
+    {
+        $before = setlocale(LC_MESSAGES, '0');
+        $found = setlocale(LC_MESSAGES, self::REAL_LOCALE);
+        setlocale(LC_MESSAGES, $before);
+        if ($found === false) {
+            $this->markTestSkipped('the locale "' . self::REAL_LOCALE . '" is not installed');
+        }
+    }
+
+    #[DataProvider('previousProcessSettings')]
+    public function testActivateKeepsARealLocaleAndRestoreLeavesItWhileTakingLanguageBack(string|false $language, string|false $locpath): void
+    {
+        MigratedPoFixture::requireGlibcLocale($this);
+        $this->requireRealLocale();
+        setlocale(LC_MESSAGES, self::REAL_LOCALE);
+        putenv($language === false ? 'LANGUAGE' : 'LANGUAGE=' . $language);
+        putenv($locpath === false ? 'LOCPATH' : 'LOCPATH=' . $locpath);
+
+        $this->activateBuild();
+
+        $this->assertTrue(NativeGettext::isActive());
+        $this->assertSame(self::REAL_LOCALE, setlocale(LC_MESSAGES, '0'));
+        $this->assertSame(NativeGettext::CATALOG_LANGUAGE, getenv('LANGUAGE'));
+        $this->assertSame($locpath, getenv('LOCPATH'), 'LOCPATH is not touched when the locale stays');
+        $this->assertSame('Hallo', NativeGettext::translate(self::DOMAIN, 'greeting'));
+
+        NativeGettext::restore();
+
+        $this->assertFalse(NativeGettext::isActive());
+        $this->assertSame(self::REAL_LOCALE, setlocale(LC_MESSAGES, '0'), 'restore() leaves a locale it did not set');
+        $this->assertSame($language, getenv('LANGUAGE'));
+        $this->assertSame($locpath, getenv('LOCPATH'));
+    }
+
+    #[DataProvider('lcAllBeforeSystemLocaleActivation')]
+    public function testSystemLocaleBranchKeepsARealLocaleSetsLcAllToItAndRestoreGivesLcAllBack(string|false $lc_all): void
+    {
+        $this->requireRealLocale();
+        NativeGettext::useSystemLocaleForTests(true);
+        setlocale(LC_MESSAGES, self::REAL_LOCALE);
+        putenv($lc_all === false ? 'LC_ALL' : 'LC_ALL=' . $lc_all);
+
+        $this->activateSystemLocale();
+
+        $this->assertSame(self::REAL_LOCALE, setlocale(LC_MESSAGES, '0'));
+        $this->assertSame(self::REAL_LOCALE, getenv('LC_ALL'));
+        $this->assertTrue(NativeGettext::isActive());
+        $this->assertSame('Hallo', NativeGettext::translate(self::DOMAIN, 'greeting'));
+
+        NativeGettext::restore();
+
+        $this->assertSame($lc_all, getenv('LC_ALL'));
+        $this->assertSame(self::REAL_LOCALE, setlocale(LC_MESSAGES, '0'));
+        $this->assertFalse(NativeGettext::isActive());
+    }
+
+    /**
+     * @return iterable<string, array{bool}>
+     */
+    public static function branches(): iterable
+    {
+        yield 'glibc branch (own locale)' => [false];
+        yield 'system locale branch' => [true];
+    }
+
+    #[DataProvider('branches')]
+    public function testReactivateIfLostKeepsALocaleOtherCodeSetForEverything(bool $system_locale): void
+    {
+        $system_locale ? $this->requireSystemLocale() : MigratedPoFixture::requireGlibcLocale($this);
+        $this->requireRealLocale();
+        NativeGettext::useSystemLocaleForTests($system_locale);
+        $this->activateBuildInBranch($system_locale);
+        $locpath = getenv('LOCPATH');
+        setlocale(LC_ALL, self::REAL_LOCALE);
+        $this->assertFalse(NativeGettext::isActive(), 'precondition');
+
+        $this->assertTrue(NativeGettext::reactivateIfLost());
+
+        $this->assertTrue(NativeGettext::isActive());
+        $this->assertSame(self::REAL_LOCALE, setlocale(LC_MESSAGES, '0'));
+        $this->assertSame($locpath, getenv('LOCPATH'));
+        $this->assertSame('Hallo', NativeGettext::translate(self::DOMAIN, 'greeting'));
+        $this->assertFalse(NativeGettext::reactivateIfLost(), 'nothing left to repair');
+
+        NativeGettext::restore();
+        $this->assertSame(self::REAL_LOCALE, setlocale(LC_MESSAGES, '0'), 'the locale of the other code is not ours to take back');
+    }
+
+    private function activateBuildInBranch(bool $system_locale): void
+    {
+        $system_locale ? $this->activateSystemLocale() : $this->activateBuild();
+    }
+
+    public function testActivateStillFailsWithoutLocaleDirectoryWhenARealLocaleStays(): void
+    {
+        MigratedPoFixture::requireGlibcLocale($this);
+        $this->requireRealLocale();
+        setlocale(LC_MESSAGES, self::REAL_LOCALE);
+        putenv('LANGUAGE=de');
+        $this->assertDirectoryDoesNotExist($this->root . '/locale-none');
+
+        $this->assertFalse(NativeGettext::activate($this->root . '/locale-none'));
+
+        $this->assertStringContainsString(NativeGettext::LOCALE_NAME, (string) NativeGettext::getProblem());
+        $this->assertFalse(NativeGettext::isActive());
+        $this->assertSame(self::REAL_LOCALE, setlocale(LC_MESSAGES, '0'));
+        $this->assertSame('de', getenv('LANGUAGE'));
+    }
+
+    /**
+     * @return iterable<string, array{string}>
+     */
+    public static function localesThatAreNotReal(): iterable
+    {
+        yield 'C.UTF-8' => ['C.UTF-8'];
+        yield 'C.utf8' => ['C.utf8'];
+        yield 'POSIX' => ['POSIX'];
+        yield 'C' => ['C'];
+    }
+
+    #[DataProvider('localesThatAreNotReal')]
+    public function testALocaleOfTheCFamilyIsNotRealSoTheOwnLocaleIsActivatedAndTakenBackAgain(string $locale): void
+    {
+        MigratedPoFixture::requireGlibcLocale($this);
+        if (setlocale(LC_MESSAGES, $locale) === false) {
+            $this->markTestSkipped('the locale "' . $locale . '" is not installed');
+        }
+        $before = setlocale(LC_MESSAGES, '0');
+
+        $this->activateBuild();
+
+        $this->assertSame(NativeGettext::LOCALE_NAME, setlocale(LC_MESSAGES, '0'));
+        $this->assertSame('Hallo', NativeGettext::translate(self::DOMAIN, 'greeting'));
+
+        NativeGettext::restore();
+
+        $this->assertSame($before, setlocale(LC_MESSAGES, '0'));
+    }
+
+    public function testTheOwnLocaleIsNeverTakenForARealOneAfterLanguageWasLost(): void
+    {
+        MigratedPoFixture::requireGlibcLocale($this);
+        $before = setlocale(LC_MESSAGES, '0');
+        $this->activateBuild();
+        putenv('LANGUAGE');
+        $this->assertFalse(NativeGettext::isActive(), 'precondition');
+
+        $this->assertTrue(NativeGettext::reactivateIfLost());
+
+        $this->assertSame(NativeGettext::LOCALE_NAME, setlocale(LC_MESSAGES, '0'));
+        $this->assertSame('Hallo', NativeGettext::translate(self::DOMAIN, 'greeting'));
+
+        NativeGettext::restore();
+
+        $this->assertSame($before, setlocale(LC_MESSAGES, '0'), 'the own locale is ours, so it is taken back');
+        $this->assertNotSame(NativeGettext::CATALOG_LANGUAGE, getenv('LANGUAGE'));
+    }
+
+    #[RunInSeparateProcess]
+    public function testTheIcuDefaultLocaleStaysTheOneOfTheAdministrator(): void
+    {
+        MigratedPoFixture::requireGlibcLocale($this);
+        $this->requireRealLocale();
+        if (!extension_loaded('intl')) {
+            $this->markTestSkipped('needs ext-intl');
+        }
+        setlocale(LC_ALL, [self::REAL_LOCALE]);
+        setlocale(LC_NUMERIC, 'C');
+
+        $this->activateBuild();
+
+        $this->assertSame('Hallo', NativeGettext::translate(self::DOMAIN, 'greeting'));
+        $this->assertSame('de_DE', \locale_get_default());
+    }
+
     public function testReactivateIfLostDoesNothingWithoutAnActivationOrWhileActive(): void
     {
         // setUp()'s build activated native gettext - restore() keeps that locale for a reactivation
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/OverlayDirectoryOwnershipTest.php b/components/ILIAS/Language/tests/ComponentTranslation/OverlayDirectoryOwnershipTest.php
index 978544ccc0da..240faa784ccb 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/OverlayDirectoryOwnershipTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/OverlayDirectoryOwnershipTest.php
@@ -32,6 +32,8 @@
  */
 class OverlayDirectoryOwnershipTest extends TestCase
 {
+    use RestoresProcessState;
+
     private const string MODULE = 'stest';
 
     private string $fixture_directory;
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/ShippedPoMergerTest.php b/components/ILIAS/Language/tests/ComponentTranslation/ShippedPoMergerTest.php
index 5239c90203f2..e45380fef9cf 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/ShippedPoMergerTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/ShippedPoMergerTest.php
@@ -39,6 +39,8 @@
  */
 class ShippedPoMergerTest extends TestCase
 {
+    use RestoresProcessState;
+
     private const string MODULE = 'mtest';
     private const string LANG = 'de';
 
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsBuildTest.php b/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsBuildTest.php
index 356fe38d92f9..ab4802a339fa 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsBuildTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsBuildTest.php
@@ -34,19 +34,15 @@
  */
 class ShippedTranslationsBuildTest extends TestCase
 {
+    use \RestoresProcessState;
+
     private string $root;
     private string $artifacts;
-    private string|false $messages_locale;
-    private string|false $language;
-    private string|false $lc_all;
 
     protected function setUp(): void
     {
         // never the build of the installation, see MigratedPoFixture::resetRuntime()
         MigratedPoFixture::resetRuntime();
-        $this->messages_locale = setlocale(LC_MESSAGES, '0');
-        $this->language = getenv('LANGUAGE');
-        $this->lc_all = getenv('LC_ALL');
         $this->root = sys_get_temp_dir() . '/ilias_shipped_build_test_' . bin2hex(random_bytes(6));
         mkdir($this->root, 0775, true);
         $this->artifacts = MigratedLanguageFilePaths::shippedArtifactDirectory($this->root);
@@ -54,12 +50,6 @@ protected function setUp(): void
 
     protected function tearDown(): void
     {
-        NativeGettext::resetForTests();
-        if ($this->messages_locale !== false) {
-            setlocale(LC_MESSAGES, $this->messages_locale);
-        }
-        putenv($this->language === false ? 'LANGUAGE' : 'LANGUAGE=' . $this->language);
-        putenv($this->lc_all === false ? 'LC_ALL' : 'LC_ALL=' . $this->lc_all);
         MigratedPoFixture::removeDirectory($this->root);
         MigratedPoFixture::resetRuntime();
     }
@@ -254,6 +244,49 @@ public function testABuildNativeGettextCannotServeLeavesCurrentAndThePreviousBui
         $this->assertDirectoryExists($this->buildDirectory((string) $good['build']));
     }
 
+    private function useRealLocaleForMessages(): string
+    {
+        MigratedPoFixture::requireGlibcLocale($this);
+        $before = setlocale(LC_MESSAGES, '0');
+        $real = setlocale(LC_MESSAGES, 'de_DE.UTF-8');
+        if ($real === false) {
+            setlocale(LC_MESSAGES, $before);
+            $this->markTestSkipped('the locale "de_DE.UTF-8" is not installed');
+        }
+        // as in a fresh Setup process, started with the locale of an administrator
+        NativeGettext::resetForTests();
+        NativeGettext::useSystemLocaleForTests(false);
+        setlocale(LC_MESSAGES, $real);
+
+        return $real;
+    }
+
+    public function testABuildNativeGettextCannotServeIsRefusedEvenWhenARealLocaleWasActiveBeforeAndItStaysUnchanged(): void
+    {
+        $real = $this->useRealLocaleForMessages();
+        $this->ship('mfa', 'de', ['greeting' => 'Hallo']);
+
+        try {
+            $this->build(['mfa'], $this->brokenLocaleSource());
+            $this->fail('the build must not be accepted');
+        } catch (RuntimeException $e) {
+            $this->assertStringContainsString('cannot be activated', $e->getMessage());
+        }
+
+        $this->assertSame($real, setlocale(LC_MESSAGES, '0'));
+    }
+
+    public function testAnIntactBuildIsCheckedWithItsOwnLocaleAndLeavesARealLocaleOfTheProcessUnchanged(): void
+    {
+        $real = $this->useRealLocaleForMessages();
+        $this->ship('mfa', 'de', ['greeting' => 'Hallo']);
+
+        $result = $this->build(['mfa']);
+
+        $this->assertNotNull($result['build']);
+        $this->assertSame($real, setlocale(LC_MESSAGES, '0'));
+    }
+
     public function testABuildWithoutAnyTranslatedValueCannotBeCheckedAndIsRefused(): void
     {
         $this->ship('mfa', 'de', ['greeting' => 'Hallo']);
diff --git a/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsTest.php b/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsTest.php
index ca99599517fb..1260cf3d88ec 100644
--- a/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsTest.php
+++ b/components/ILIAS/Language/tests/ComponentTranslation/ShippedTranslationsTest.php
@@ -32,6 +32,8 @@
  */
 class ShippedTranslationsTest extends TestCase
 {
+    use \RestoresProcessState;
+
     private string $root;
     private ShippedTranslations $shipped_translations;
 
diff --git a/components/ILIAS/Language/tests/MigratedPoFixture.php b/components/ILIAS/Language/tests/MigratedPoFixture.php
index 53951e22c3e1..bb2edd15bc11 100644
--- a/components/ILIAS/Language/tests/MigratedPoFixture.php
+++ b/components/ILIAS/Language/tests/MigratedPoFixture.php
@@ -37,6 +37,8 @@
  */
 final class MigratedPoFixture
 {
+    private const array PROCESS_ENVIRONMENT = ['LANGUAGE', 'LC_ALL', 'LC_MESSAGES', 'LOCPATH', 'LANG'];
+
     private static bool $artifact_cleanup_registered = false;
 
     /** The C library never forgets a catalog path it has looked at, so every resetRuntime() gets a directory of its own. */
@@ -298,6 +300,36 @@ public static function resetRuntime(?string $artifact_directory = null): void
         NativeGettext::resetForTests();
     }
 
+    /**
+     * @return array{locale: string|false, env: array<string, string|false>}
+     */
+    public static function captureProcessState(): array
+    {
+        $env = [];
+        foreach (self::PROCESS_ENVIRONMENT as $name) {
+            $env[$name] = getenv($name);
+        }
+
+        return ['locale' => setlocale(LC_ALL, '0'), 'env' => $env];
+    }
+
+    /**
+     * Gives back what captureProcessState() saw and forgets the static state of the runtime read path.
+     *
+     * @param array{locale: string|false, env: array<string, string|false>} $state
+     */
+    public static function restoreProcessState(array $state): void
+    {
+        MigratedTranslations::resetForTests();
+        NativeGettext::resetForTests();
+        foreach ($state['env'] as $name => $value) {
+            putenv($value === false ? $name : $name . '=' . $value);
+        }
+        if ($state['locale'] !== false && setlocale(LC_ALL, $state['locale']) === false) {
+            setlocale(LC_ALL, 'C');
+        }
+    }
+
     /** Moves the builds to a directory this process has not looked at: the C library keeps what it read from a path (mapped, not copied), so a file damaged in place must not be one it read. */
     public static function moveArtifactsToFreshDirectory(): void
     {
diff --git a/components/ILIAS/Language/tests/RestoresProcessState.php b/components/ILIAS/Language/tests/RestoresProcessState.php
new file mode 100644
index 000000000000..799cd8fc4929
--- /dev/null
+++ b/components/ILIAS/Language/tests/RestoresProcessState.php
@@ -0,0 +1,46 @@
+<?php
+
+/**
+ * This file is part of ILIAS, a powerful learning management system
+ * published by ILIAS open source e-Learning e.V.
+ *
+ * ILIAS is licensed with the GPL-3.0,
+ * see https://www.gnu.org/licenses/gpl-3.0.en.html
+ * You should have received a copy of said license along with the
+ * source code, too.
+ *
+ * If this is not the case or you just want to try ILIAS, you'll find
+ * us at:
+ * https://www.ilias.de
+ * https://github.com/ILIAS-eLearning
+ *
+ *********************************************************************/
+
+declare(strict_types=1);
+
+use PHPUnit\Framework\Attributes\After;
+use PHPUnit\Framework\Attributes\Before;
+
+/**
+ * Whatever a test does to the process (locale, LANGUAGE/LC_*, LOCPATH, the static state of the runtime read path),
+ * the next test starts as this one did.
+ */
+trait RestoresProcessState
+{
+    /** @var array{locale: string|false, env: array<string, string|false>}|null */
+    private ?array $process_state_before_test = null;
+
+    #[Before]
+    protected function captureProcessStateBeforeTest(): void
+    {
+        $this->process_state_before_test = MigratedPoFixture::captureProcessState();
+    }
+
+    #[After]
+    protected function restoreProcessStateAfterTest(): void
+    {
+        if ($this->process_state_before_test !== null) {
+            MigratedPoFixture::restoreProcessState($this->process_state_before_test);
+        }
+    }
+}
diff --git a/components/ILIAS/Language/tests/Setup/InstallLanguagesRemovesOverlaysTest.php b/components/ILIAS/Language/tests/Setup/InstallLanguagesRemovesOverlaysTest.php
index d08011499622..66b645d52154 100644
--- a/components/ILIAS/Language/tests/Setup/InstallLanguagesRemovesOverlaysTest.php
+++ b/components/ILIAS/Language/tests/Setup/InstallLanguagesRemovesOverlaysTest.php
@@ -35,6 +35,8 @@
  */
 class InstallLanguagesRemovesOverlaysTest extends TestCase
 {
+    use RestoresProcessState;
+
     private const string MODULE = 'pilot';
 
     private string $root;
diff --git a/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerDatabaseRowsOfMigratedModuleTest.php b/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerDatabaseRowsOfMigratedModuleTest.php
index 771ed1208103..0b0fdcb62e46 100644
--- a/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerDatabaseRowsOfMigratedModuleTest.php
+++ b/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerDatabaseRowsOfMigratedModuleTest.php
@@ -28,6 +28,8 @@
  */
 class LanguageInstallationManagerDatabaseRowsOfMigratedModuleTest extends TestCase
 {
+    use RestoresProcessState;
+
     /**
      * @param array<string, array{0: string, 1: ?string, 2: ?string}> $rows
      * @param array<string, string> $final_entries
diff --git a/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerMigratedModulesTestCase.php b/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerMigratedModulesTestCase.php
index b91559e0095d..695f4f4060bf 100644
--- a/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerMigratedModulesTestCase.php
+++ b/components/ILIAS/Language/tests/Setup/LanguageInstallationManagerMigratedModulesTestCase.php
@@ -41,6 +41,8 @@
  */
 abstract class LanguageInstallationManagerMigratedModulesTestCase extends TestCase
 {
+    use RestoresProcessState;
+
     protected const string NOW = '2026-01-02 03:04:05';
     protected const string MODULE = 'pilot';
 
diff --git a/components/ILIAS/Language/tests/Setup/ShippedLanguageFilesCompiledObjectiveTest.php b/components/ILIAS/Language/tests/Setup/ShippedLanguageFilesCompiledObjectiveTest.php
index c1bf4adcfe53..dcfc391c2f07 100644
--- a/components/ILIAS/Language/tests/Setup/ShippedLanguageFilesCompiledObjectiveTest.php
+++ b/components/ILIAS/Language/tests/Setup/ShippedLanguageFilesCompiledObjectiveTest.php
@@ -33,6 +33,8 @@
  */
 class ShippedLanguageFilesCompiledObjectiveTest extends TestCase
 {
+    use RestoresProcessState;
+
     private string $root;
 
     protected function setUp(): void
diff --git a/components/ILIAS/Language/tests/Setup/ilLanguagesInstalledAndUpdatedObjectiveTest.php b/components/ILIAS/Language/tests/Setup/ilLanguagesInstalledAndUpdatedObjectiveTest.php
index 6435c58623ee..ca32727eb128 100644
--- a/components/ILIAS/Language/tests/Setup/ilLanguagesInstalledAndUpdatedObjectiveTest.php
+++ b/components/ILIAS/Language/tests/Setup/ilLanguagesInstalledAndUpdatedObjectiveTest.php
@@ -35,6 +35,8 @@
  */
 class ilLanguagesInstalledAndUpdatedObjectiveTest extends TestCase
 {
+    use RestoresProcessState;
+
     private bool $had_global_db;
     private mixed $previous_global_db = null;
 
diff --git a/components/ILIAS/Language/tests/WithPluralFormKeysTest.php b/components/ILIAS/Language/tests/WithPluralFormKeysTest.php
index 41377d5dc108..25b4189e53cc 100644
--- a/components/ILIAS/Language/tests/WithPluralFormKeysTest.php
+++ b/components/ILIAS/Language/tests/WithPluralFormKeysTest.php
@@ -26,6 +26,8 @@
  */
 class WithPluralFormKeysTest extends TestCase
 {
+    use RestoresProcessState;
+
     private const string SEPARATOR = '#:#';
 
     /**
diff --git a/components/ILIAS/Language/tests/ilLanguageBaseTestCase.php b/components/ILIAS/Language/tests/ilLanguageBaseTestCase.php
index bd24f0f574a5..9c304fc6265b 100644
--- a/components/ILIAS/Language/tests/ilLanguageBaseTestCase.php
+++ b/components/ILIAS/Language/tests/ilLanguageBaseTestCase.php
@@ -29,6 +29,8 @@
  */
 abstract class ilLanguageBaseTestCase extends TestCase
 {
+    use RestoresProcessState;
+
     protected function setUp(): void
     {
         // never the build of the installation, see MigratedPoFixture::resetRuntime()
diff --git a/components/ILIAS/Language/tests/ilObjLanguageExtGUIDispatchTest.php b/components/ILIAS/Language/tests/ilObjLanguageExtGUIDispatchTest.php
index 29093a771777..e4b1edbe1bfc 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageExtGUIDispatchTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageExtGUIDispatchTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -24,6 +24,8 @@
 
 class ilObjLanguageExtGUIDispatchTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageGuiTestSupport;
 
     private const array COMMANDS = [
diff --git a/components/ILIAS/Language/tests/ilObjLanguageExtGUIMaintenanceTest.php b/components/ILIAS/Language/tests/ilObjLanguageExtGUIMaintenanceTest.php
index 9d6cb20b97e3..681fd03347e1 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageExtGUIMaintenanceTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageExtGUIMaintenanceTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -28,6 +28,8 @@
 
 class ilObjLanguageExtGUIMaintenanceTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageExtGUITestFixture;
 
     private ?string $maintenance_directory = null;
diff --git a/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveNewEntryTest.php b/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveNewEntryTest.php
index d3f0478443ff..ba8df6a0e54e 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveNewEntryTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveNewEntryTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -27,6 +27,8 @@
 
 class ilObjLanguageExtGUISaveNewEntryTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageExtGUITestFixture;
 
     /** @return ilObjLanguageExtGUI&MockObject */
diff --git a/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveObjectTest.php b/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveObjectTest.php
index d7d1404e0706..8513b5ea7637 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveObjectTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveObjectTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -25,6 +25,8 @@
 
 class ilObjLanguageExtGUISaveObjectTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageExtGUITestFixture;
 
     /** @var array<string, ilLanguageFile>|null */
diff --git a/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveSettingsTest.php b/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveSettingsTest.php
index 3b01dc06257c..5497ed7c5b9d 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveSettingsTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageExtGUISaveSettingsTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -27,6 +27,8 @@
 
 class ilObjLanguageExtGUISaveSettingsTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageExtGUITestFixture;
 
     /** @return ilObjLanguageExtGUI&MockObject */
diff --git a/components/ILIAS/Language/tests/ilObjLanguageExtGUITest.php b/components/ILIAS/Language/tests/ilObjLanguageExtGUITest.php
index 4c00063644fb..9b7c5b229554 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageExtGUITest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageExtGUITest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -23,6 +23,8 @@
 
 class ilObjLanguageExtGUITest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageExtGUITestFixture;
 
     /** A data-changing command missing here would be triggerable via GET without the CSRF token. */
diff --git a/components/ILIAS/Language/tests/ilObjLanguageExtGUIUploadTest.php b/components/ILIAS/Language/tests/ilObjLanguageExtGUIUploadTest.php
index 4d8942a93d34..27e5f25914b1 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageExtGUIUploadTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageExtGUIUploadTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -28,6 +28,8 @@
  */
 class ilObjLanguageExtGUIUploadTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageExtGUITestFixture;
 
     /**
diff --git a/components/ILIAS/Language/tests/ilObjLanguageFolderGUIIdValidationTest.php b/components/ILIAS/Language/tests/ilObjLanguageFolderGUIIdValidationTest.php
index 64bc0ea3f10f..2ae8b9e2cc19 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageFolderGUIIdValidationTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageFolderGUIIdValidationTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -27,6 +27,8 @@
 
 class ilObjLanguageFolderGUIIdValidationTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageFolderGUITestFixture;
 
     /** One non-"lng" id aborts the whole request: failure message, redirect to "view", InstallLanguage never called. */
diff --git a/components/ILIAS/Language/tests/ilObjLanguageFolderGUILanguageDetectionTest.php b/components/ILIAS/Language/tests/ilObjLanguageFolderGUILanguageDetectionTest.php
index e0db4dc88b59..135b91634cfd 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageFolderGUILanguageDetectionTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageFolderGUILanguageDetectionTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -30,6 +30,8 @@
 #[AllowMockObjectsWithoutExpectations]
 class ilObjLanguageFolderGUILanguageDetectionTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageFolderGUITestFixture;
 
     /**
diff --git a/components/ILIAS/Language/tests/ilObjLanguageFolderGUITest.php b/components/ILIAS/Language/tests/ilObjLanguageFolderGUITest.php
index f3255fa5cc51..7334727004e4 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageFolderGUITest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageFolderGUITest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -28,6 +28,8 @@
 
 class ilObjLanguageFolderGUITest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageFolderGUITestFixture;
 
     /** @return ilObjLanguageFolderGUI&\PHPUnit\Framework\MockObject\MockObject */
diff --git a/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallChangesTest.php b/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallChangesTest.php
index 3716eac8ddef..e37c054e0141 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallChangesTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallChangesTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -25,6 +25,8 @@
 
 class ilObjLanguageFolderGUIUninstallChangesTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageFolderGUITestFixture;
 
     public function testUninstallChangesObjectEmbedsTheThrowableMessageFromAnErrorResultIntoTheFailureMessage(): void
diff --git a/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallTest.php b/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallTest.php
index 82d7dabcb564..2f84a52a18cf 100644
--- a/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallTest.php
+++ b/components/ILIAS/Language/tests/ilObjLanguageFolderGUIUninstallTest.php
@@ -6,8 +6,8 @@
  *
  * ILIAS is licensed with the GPL-3.0,
  * see https://www.gnu.org/licenses/gpl-3.0.en.html
- * You should have received a copy of said license along with
- * the source code, too.
+ * You should have received a copy of said license along with the
+ * source code, too.
  *
  * If this is not the case or you just want to try ILIAS, you'll find
  * us at:
@@ -25,6 +25,8 @@
 
 class ilObjLanguageFolderGUIUninstallTest extends TestCase
 {
+    use RestoresProcessState;
+
     use ilObjLanguageFolderGUITestFixture;
 
     public function testUninstallObjectEmbedsTheThrowableMessageFromAnErrorResultIntoTheFailureMessage(): void