This can be so frustrating, even more so, in my opinion, if you also speak their mother tongue. I, for example, speak both Spanish and Galician, in addition to English. I therefore find it difficult to discourage students as much as I should from speaking their native language, because I still understand everything that they say. I sometimes think that if I understand, there is every reason to just move on with the class to avoid wasting time on correction or admonishment. But this is a failure on my part, speaking their own language means that they feel more and more comfortable not making an effort when trying to express themselves. They may take to using more of their own language mixed in with English, or simply using their own language all of the time. It’s a very difficult precedent to turn around.
Make Them Understand The Problem – Be sure to let them know how much it damages their learning to constantly speak in their native tongue. Denying themselves the opportunity to improve through practise and correction.
Use the native language appropriately – In my personal opinion, it can occasionally be useful to speak to students in their native language ad vice versa, when and if you can also express yourself in that language. Especially with adults. If, for example, you are trying to explain a phrase or a concept that is particularly difficult for them to grasp, translating it might be easier. Saving time and a lot of effort in order to keep the rhythm and the flow of the lesson.
Be strict! – Try the somewhat less subtle tactic of simply ignoring students when they speak in their native language, or intimate that you don’t understand. Forcing them to use English!
Use Exam Guidelines and Practice – By continuously reminding students of exam requirements, native language use may be kept to a minimum. Take the Cambridge suite of exams as an example, examiners take a dim view of any other language being used in an exam. Hesitation, or repetition is also not encouraged, but is definitely preferable. So get students to do exam practice and assign them rough scores, they will see that using their first language will damage their results. Get them to make the extra effort to think of the word in English, even if they need to hesitate to do so.
Anything to add? Let me know in the comment section!