I would argue...none!! Even for absolute beginners. I teach English to students from all over the world, many of whom are at a pre-beginning level, and I definitely can't speak most of their languages. Pictures, picture dictionaries, flashcards, realia, and any other authentic materials are extremely important aids, but it is actually better if you don't speak any of the learners' language. It is challenging and requires a lot of prep, but one nice thing about teaching beginners is that it is mostly just tangible nouns and verbs, all of which can be acted out or displayed quite easily. I think it gets harder around the high beginning level when you start dealing with intangibles, concepts, and tricky grammar. But it is still possible and, in my opinion, actually preferable.
That being said, one huge advantage of knowing your learners' native language is that you can use it to help you explain English in terms they will understand. A very simple example: in Mandarin there is no difference in the pronunciation of the pronouns he, she and it. So, when I'm teaching these for the first time to my Chinese students, I point this out and emphasize the ridiculousness of calling a man "she" and vice versa, so that they realize that this is something different that they will have to pay close attention to when speaking English.
However, I am a strong advocate of using your knowledge of foreign languages only to help augment your teaching of English, and not to actually translate or use it in class.