[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>