General curtesy is that if you are speaking your language in a group of  ppl where thesr is a presence of ppl who dont understand your language you first ask/inform other guys that you are going to talk with your friends in your language. If you dont do that it is offensive as nepalinetwork says.
and even after doing that,  you can speak in your language for only short period of time. If your conversation is longer then again it is offensive.