[BUGFIX] Set typo3Language for new site languages if possible

If a sys_language record is properly filled with the language
information, the site language record can be filled as well.

Setting the typo3Language by default with the correct languages makes it
easier to setup languages.

Resolves: #92422
Releases: master, 10.4
Change-Id: I3c9e2322cd0aa3963ede36586cc97babe0f003a8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65897
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>

typo3/sysext/backend/Classes/Controller/SiteInlineAjaxController.php

@@ -27,6 +27,7 @@
 use TYPO3\CMS\Core\Database\ConnectionPool;
 use TYPO3\CMS\Core\Database\Query\Restriction\HiddenRestriction;
 use TYPO3\CMS\Core\Http\JsonResponse;
+use TYPO3\CMS\Core\Localization\Locales;
 use TYPO3\CMS\Core\Utility\ArrayUtility;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
 use TYPO3\CMS\Core\Utility\MathUtility;
@@ -98,6 +99,12 @@ public function newInlineChildAction(ServerRequestInterface $request): ResponseI
                 if (!empty($row['language_isocode'])) {
                     $defaultDatabaseRow['iso-639-1'] = $row['language_isocode'];
                     $defaultDatabaseRow['base'] = '/' . $row['language_isocode'] . '/';
+
+                    $locales = GeneralUtility::makeInstance(Locales::class);
+                    $allLanguages = $locales->getLanguages();
+                    if (isset($allLanguages[$row['language_isocode']])) {
+                        $defaultDatabaseRow['typo3Language'] = $row['language_isocode'];
+                    }
                 }
                 if (!empty($row['flag']) && $row['flag'] === 'multiple') {
                     $defaultDatabaseRow['flag'] = 'global';