[BUGFIX] Flush l10n cache after downloading languages

This patch let's the Maintenance Area flush the `l10n` caches after
language packs were downloaded. Since the Install Tool uses
"hacked settings", we have to create a new instance of the `CacheManager`
to be able to operate on the mentioned cache. The same handling can be
seen in `ClearCacheService::clearAll()`.

Resolves: #84645
Releases: master
Change-Id: I221bd0d77f2991eac550316586002a8329bb36df
Reviewed-on: https://review.typo3.org/56583
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joshua Westerheide <account@jdoubleu.de>
Tested-by: Joshua Westerheide <account@jdoubleu.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>

typo3/sysext/install/Classes/Controller/MaintenanceController.php

@@ -680,6 +680,17 @@ public function languagePacksUpdateIsoTimesAction(ServerRequestInterface $reques
         $isos = $request->getParsedBody()['install']['isos'];
         $languagePackService = GeneralUtility::makeInstance(LanguagePackService::class);
         $languagePackService->setLastUpdatedIsoCode($isos);
+
+        // The cache manager is already instantiated in the install tool
+        // with some hacked settings to disable caching of extbase and fluid.
+        // We want a "fresh" object here to operate on a different cache setup.
+        // cacheManager implements SingletonInterface, so the only way to get a "fresh"
+        // instance is by circumventing makeInstance and/or the objectManager and
+        // using new directly!
+        $cacheManager = new \TYPO3\CMS\Core\Cache\CacheManager();
+        $cacheManager->setCacheConfigurations($GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations']);
+        $cacheManager->getCache('l10n')->flush();
+
         return new JsonResponse(['success' => true]);
     }