Just came across this. If you get a language not allowed error in the app when trying to post, it could be a mismatch between the language in the user profile and the community language. I discovered this while trying to comment on a community I am a mod in and kept getting denied (my profile language remains ‘undefined’ and the community had English selected only - guess I got overzealous).
My takeaway from this is to keep the ‘undefined’ language tag in communities just to be on the safe side.
lesson learned for me… the hard way
And to add, those two communities have languages set because I want the first one to be purely in Finnish and the second one purely in English. This mirrors how it is on Reddit.