Thanks, these are some helpful comments.
Our main reasons for considering 2 languages in 1 forum are to reduce complexity and have some interaction (at least from German speakers who also understand English, probably not so much the other way round).
But at the same time, we know from our website that English speakers are more likely to leave if there are many German elements included on the site they look at. So by having very clear EN vs DE categories, this problem could be reduced because EN users just need to click on their language once and they are in an English environment. But I understand your point that it might also reduce interaction, like German users writing in the English part of the forum.
We’ll think about this some more and try to look at other examples.