Fix behavior of multi-language editing
As realized in #6199 multi-language editing does not work correctly yet.
Adding >2 languages, deletes one of the old descriptions so that always only 2 languages remain.
This happens only when saving, while an unsaved TextData shows all existing translations.
Also the behavior of the Translation editor is strange. With a choosen language one would expect that this language is shown on the right side after opening the Editor, but it always shows no language. One first has to choose the language to translate.
Also it is currently impossible to delete a language.
Partly these issues have been solved for the Term Editor already, but there the data structure is a bit different.