Hi Philipp,
Both in traditional and reactive/mobile, what should happen is the other way around: if a more specific translation is not found the translation of a parent locale is given (e.g. if locale is set to "de-DE" and there is no translation there, then the translation for "de" is returned). If this is not working correctly please contact OutSystems support.
BTW, if you want to move translations to a different locale you can always do that by exporting translations to an excel file, do the bulk changes on excel and then importing them again.
Cheers,
Tiago Simões