[BUGFIX] Respect availableLanguages from AdditionalConfiguration.php

if $GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['lang']['availableLanguages']
is set in AdditionalConfiguration.php, the language module will now 
respect that setting.

Resolves: #78876
Releases: master
Change-Id: Iec6a0f05db835ae949fbacdc2cdf500a0950d57c
Reviewed-on: https://review.typo3.org/50871


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>