[BUGFIX] Fix languageUpdate signal check in setup module

Since #93663 the setup module uses the be_users.lang field
for the UI language. Therefore, when switching languages, the
corresponding value is not longer found on the root level of
the form data array, but in the `be_users` subarray.

Resolves: #93756
Releases: master
Change-Id: Ib593f969a61ddfc72030c21a85ba3b3ab401d181
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68467
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/setup/Classes/Controller/SetupModuleController.php

@@ -224,7 +224,7 @@ protected function storeIncomingData(array $postData)
             $save_before = md5(serialize($backendUser->uc));
             // PUT SETTINGS into the ->uc array:
             // Reload left frame when switching BE language
-            if (isset($d['lang']) && $d['lang'] !== $backendUser->user['lang']) {
+            if (isset($d['be_users']['lang']) && $d['be_users']['lang'] !== $backendUser->user['lang']) {
                 $this->languageUpdate = true;
             }
             // Reload pagetree if the title length is changed