Often when you teach a language to students, you won't know their language. I've taught classrooms in US where I have Arabic speakers, Chinese speakers, Turkish speakers, Spanish speakers etc. all in the same class. I don't speak most of those languages whatsoever and there aren't translators on hand. If students are above a basic level, this works fine. You can explain English language concepts to students even if they have a limited vocabulary. If they know no English at all its an issue, then they'll need to utilize more bilingual materials or have a bilingual teacher.

Same is true teaching abroad, but usually you're working with students who share a common language. Therefore schools usually have bilingual teachers from that country in addition to native speaking teachers. Worse schools/teachers might mostly just utilize a foreign face that doesn't do much besides speak natively accented English, better schools actually have foreign teachers who are certified to teach or have experience teaching.