[TASK] Clean up Language-related functionality

This change cleans up
some tests, and also streamlines
some usages (not all of them) of
$TYPO3_CONF_VARS['EXTCONF']['lang']['availableLanguages'].

Resolves: #99578
Releases: main
Change-Id: I52b6f43260d08abf59d6078498511084469b2308
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77423


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>