what knotical said. Make sure you understand, and can be easily understood in the language before you try to teach something in it. Knowing the terms will definitely help, but I think that's probably the last thing you'll need. I can explain scuba-related concepts in english (though I don't), but don't ask me to even try to explain them in French (though I do consider myself to be fairly competent with the language).
Here's my experience with instructors using their second language: I did my OW course in english, but my checkout dives were conducted with french-speaking instructors (they could speak english, but the one who was doing the most talking was more comfortable in french) and since almost everyone in the group was ok with it, most of the talks were in french, with an english version when somebody needed it. The english versions were interesting though, when the instructor didn't know the english word they needed and had to throw in the french term instead, like détenteur or cagoule (regulator and hood), which have little or no resemblance to the english term.
When I did a discover scuba, english was obviously not the intructor's first language. As much as I liked the guy, I decided then and there that I wasn't going to take OW with him. He seemed great in the water, but I wanted to be able to easily understand someone when they're explaining why holding your breath is a bad idea.