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.
Log on to your instance via browser (not the app) and it should be in your profile settings. ‘undefined’ is at the top of the list.
Edit: looked closer at your linked post. I see there is only English in your list.
Maybe it’s something in the way the instance was configured? That list should be populated with all languages.
Edit 2: spelling