[BUGFIX] Reimplement custom BE languages

[BUGFIX] Reimplement custom BE languages

Because of a regression the backend does not load custom BE languages.
The loading order is changed so that custom languages are available
again.

Resolves: #87101
Releases: master, 9.5
Change-Id: I0530957abc49f775497d9644047cbb49ceffcda9
Reviewed-on: https://review.typo3.org/59068
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>