Speak in English only if you are comfortable in that language.Else use your native language for your conversations.Show how friendly and strict you can be like two sides of a same coin.Be expressive and clear : I think that's what any interviewer (who will mostly be a teacher) will look at to recruit you as a teacher. No necessarily all time.You should start with your native language and later talk in English (this can prove your English skills in front of your interviewers). Hope it helps.

Be prepared to speak English if you are going for a job where you will be teaching English. You also need to know a lot about what you will be teaching, what methods you have of teaching your subject , the types of classroom management strategies you employ. You need to be familiar with the policies of the school you will be teaching at. If you have worked as a teacher before, I suggest that you take examples of students' work with you to your interview and any references or commendations you have received.

What will you be teaching? I think you probably need to speak in the language of instruction so that you are fluent in it.