I agree. My point is that if a translation isn’t high quality in some way, it’s unprofessional. The basic question is the same: if it’s not possible to make a full high quality translation, is it worth keeping or making at all? That’s a common problem in Open Source software. Translations are made by volunteers and usually lack behind the UI development. The KiCad developers have decided that incomplete translations are worth keeping because they may help some users. This of course requires that what is in the translation is correct, even though something has been left untranslated.
Personally I feel that incomplete translation (even when what’s translated is high quality) gives an unprofessional impression. No commercial software does that, I don’t remember ever seeing a half-translated big commercial program. But I can’t speak for those who don’t know English. If they really find incomplete translations useful, my personal feeling doesn’t matter at all. You should do a scientific research about the subject first, proving that German users don’t need a half made translation. Maybe the developers would listen to you then.