[BUGFIX] Enable fallback functionality for site handling

Enable the functionality again to have a site with three
languages to work in fallback mode (sys_language_mode = content_fallback)

Resolves: #86031
Releases: master
Change-Id: Id1a8897de057fb3d5a3244b3842f8dee377a4df6
Reviewed-on: https://review.typo3.org/58077
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>

typo3/sysext/core/Classes/Site/Entity/SiteLanguage.php

@@ -151,7 +151,7 @@ public function __construct(int $languageId, string $locale, UriInterface $base,
             $this->fallbackType = $attributes['fallbackType'];
         }
         if (!empty($attributes['fallbacks'])) {
-            $this->fallbackLanguageIds = $attributes['fallbacks'];
+            $this->fallbackLanguageIds = is_array($attributes['fallbacks']) ? $attributes['fallbacks'] : explode(',', $attributes['fallbacks']);
         }
         if (isset($attributes['enabled'])) {
             $this->enabled = (bool)$attributes['enabled'];