Is this just a guess of you or has there been a statement regarding posts in non-english language boards?
Because in the Airdrop rules it is explicitly mentioned that posting on non-english boards is accepted.
From my observation, I don't think people get banned for talking in other languages. But I could see how someone struggling to speak English could be seen as a low quality posts.
I would suggest literally ending every post by saying, "Warning: Sorry for my English is not my native language." Since it seems that 75% of people get banned for unknown reasons.