I point out (non English) chatter, without going overboard to the point where it affects flow. In other words – don’t spend a whole lot of time and energy trying to achieve this goal overnight, but gently push kids in the right direction.
Students can also be trained to catch each other out. If you use the phrases “English only please!”, or “Don’t speak Japanese, please!”, your students will start using them too with a little encouragement. It can also help to periodically get an assistant to address your students and their parents in their native language about what you are doing and why it is important.