[BUGFIX] Only English shown as BE language in Composer mode

Using the legacy path for the l10n files folder does not work anymore in
Composer mode, as the folder was moved into `var/` there.

Change-Id: I61dea0e4e909c5640e776e558ca493015a539015
Resolves: #86546
Releases: master
Reviewed-on: https://review.typo3.org/58560


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>