[BUGFIX] Properly handle non-existing language key

Resolves: #61999
Releases: master
Change-Id: Iebff3ed402f952c361b25593fe4aaac89d2ec149
Reviewed-on: http://review.typo3.org/33101
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Krzysztof Adamczyk <k.adamczyk@macopedia.pl>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>