[BUGFIX] Fill alternative language keys correctly

This change adds the language dependencies individually to the array
instead of completely overwriting them as a string. Since the
dependencies are iterated in the core, this function only adds to the
array instead of overwriting the first index.

Resolves: #88045
Releases: master, 9.5
Change-Id: I3101bf8f9873ffa5ceeee3cc9e9ace36858f39a4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61406
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>

typo3/sysext/extbase/Classes/Utility/LocalizationUtility.php

@@ -199,15 +199,15 @@ protected static function getLanguageKeys(): array
             } elseif (isset($tsfe->config['config']['language'])) {
                 $languageKeys['languageKey'] = $tsfe->config['config']['language'];
                 if (isset($tsfe->config['config']['language_alt'])) {
-                    $languageKeys['alternativeLanguageKeys'] = $tsfe->config['config']['language_alt'];
+                    $languageKeys['alternativeLanguageKeys'][] = $tsfe->config['config']['language_alt'];
                 }
             }
 
             if (empty($languageKeys['alternativeLanguageKeys'])) {
                 $locales = GeneralUtility::makeInstance(Locales::class);
                 if (in_array($languageKeys['languageKey'], $locales->getLocales())) {
                     foreach ($locales->getLocaleDependencies($languageKeys['languageKey']) as $language) {
-                        $languageKeys['alternativeLanguageKeys'] = $language;
+                        $languageKeys['alternativeLanguageKeys'][] = $language;
                     }
                 }
             }