For the thrill of the moment at least they'll complete it in English. Bad habits die hard however and are best dealt with from the start. Be tough with the students. Don't speak to them in their native language. When students don't correctly use them just point to the phrases and wait them out.
Most students are very uncomfortable with a sudden hush.
There should be zero L1 in the classroom. But don't translate for them, engage them in meaningful dialogue. When you are speaking in L1 for the sake of efficiency, then worry about wasting the students' time because that is exactly what you are doing. In an EFL situation, this kind of student- teacher exchange may be the only chance they have of ever communicating directly with a native speaker. After all didn't their teacher demonstrate to them on a daily basis that L1 is what is used for the true grit of conveying ideas.