To actually teach you don't need to know the local language. These days classes are taught almost entirely in the Target Language which means you introduce, explain and practice in English.

This is called Full Immersion and essentially you are trying to put the student in the same kind of linguistic environment that a native speaker learns their own language.

In fact, it does the students good. To communicate they MUST speak English and that's what it's all about.

Of course knowing a bit of the local language helps when you're outside school (e.g. shopping, ordering in a bar, etc) but you'll soon pick up the basics and it's rare that you'll find yourself living in a town where no one speaks any English at all! :)