[BUGFIX] Include missing LL file in form flexform

In patch #94429 inclusion of LL file has been removed accidentally.
To prevent multiple inclusion of LL-file in loop, incusion is done
in DataStructureIdentifierHook::__construct now.
Adds new constant for LLFile.

Resolves: #90784
Related: #94429
Releases: master
Change-Id: I4446ed6e05b8e506279e77fae4703a817831129d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70465
Tested-by: core-ci <typo3@b13.com>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Rudy Gnodde <rudy@famouswolf.com>
Reviewed-by: waldhacker <hello@waldhacker.dev>

typo3/sysext/form/Classes/Hooks/DataStructureIdentifierHook.php

@@ -44,12 +44,16 @@
  */
 class DataStructureIdentifierHook
 {
-
     /**
      * Localisation prefix
      */
     const L10N_PREFIX = 'LLL:EXT:form/Resources/Private/Language/Database.xlf:';
 
+    public function __construct()
+    {
+        $this->getLanguageService()->includeLLFile('EXT:form/Resources/Private/Language/Database.xlf');
+    }
+
     /**
      * The data structure depends on a current form selection (persistenceIdentifier)
      * and if the field "overrideFinishers" is active. Add both to the identifier to

typo3/sysext/form/Tests/Unit/Hooks/DataStructureIdentifierHookTest.php

@@ -20,6 +20,7 @@
 use Prophecy\Argument;
 use TYPO3\CMS\Core\Cache\CacheManager;
 use TYPO3\CMS\Core\Cache\Frontend\FrontendInterface;
+use TYPO3\CMS\Core\Localization\LanguageService;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
 use TYPO3\CMS\Form\Hooks\DataStructureIdentifierHook;
 use TYPO3\CMS\Form\Mvc\Persistence\FormPersistenceManagerInterface;
@@ -48,6 +49,8 @@ public function setUp(): void
         $cacheProphecy->get(Argument::cetera())->willReturn(false);
         $cacheProphecy->set(Argument::cetera())->willReturn(false);
         GeneralUtility::setSingletonInstance(CacheManager::class, $cacheManagerProphecy->reveal());
+        $languageService = $this->prophesize(LanguageService::class);
+        $GLOBALS['LANG'] = $languageService->reveal();
     }
 
     /**