but if a person has a post history where he only insists to speak in English on a local board, what do the rules of bitcointalk say in this regard?
this will entirely depend on the local board's rule. anyway, I don't know about other local boards, but in the Pilipinas board, users can create threads or reply to someone using English. I guess the fact that English is our second language is probably the reason why it is allowed(not really sure if it is the reason).