[BUGFIX] Allow fallback to default language in site config

Resolves: #86597
Releases: master
Change-Id: I119a83a76bb3b5695965490434daef78245c27dd
Reviewed-on: https://review.typo3.org/58698


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jasmina Ließmann <code@frauliessmann.de>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>