diff --git a/typo3/sysext/extbase/Classes/Utility/LocalizationUtility.php b/typo3/sysext/extbase/Classes/Utility/LocalizationUtility.php
index 3a7027ec2936..40d9e11ad565 100644
--- a/typo3/sysext/extbase/Classes/Utility/LocalizationUtility.php
+++ b/typo3/sysext/extbase/Classes/Utility/LocalizationUtility.php
@@ -199,7 +199,7 @@ 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'];
                 }
             }
 
@@ -207,7 +207,7 @@ protected static function getLanguageKeys(): array
                 $locales = GeneralUtility::makeInstance(Locales::class);
                 if (in_array($languageKeys['languageKey'], $locales->getLocales())) {
                     foreach ($locales->getLocaleDependencies($languageKeys['languageKey']) as $language) {
-                        $languageKeys['alternativeLanguageKeys'] = $language;
+                        $languageKeys['alternativeLanguageKeys'][] = $language;
                     }
                 }
             }