[TASK] EXT:form - add unit tests for the translation service

This patch adds unit tests and fixes a small bug.

Resolves: #79562
Releases: master
Change-Id: If71015177c4a36acde0b5a7a8e502b98fb841311
Reviewed-on: https://review.typo3.org/51480


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Daniel Lorenz <daniel.lorenz@extco.de>
Reviewed-by: Daniel Lorenz <daniel.lorenz@extco.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>