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.
I ran in to the same problem trying to comment on a thread. How do you get “undefined” in your language list? I only have English
https://reddthat.com/post/1976
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