Insist that students do their task in English - Shorten tasks for the sake of production if necessary
It may not appear realistic to prevent students from using their own language completely, but they should understand that during the performance of a task (if not at the planning stage - when they may need their mother tongue to ask for the new language) they must use English. If you've not discussed this at the beginning of the course, it may be a good idea to talk about the importance of this issue as soon as possible.
One thing to remember about this is how students are always tempted to use their native language when a task is hard, so do not be afraid to shorten or simplify the task for the sake of production when necessary.