I have taught in Greece, China and now Italy and I speak minimal Greek, Chinese and Italian!
I've never found it a hindrance when teaching ESL, if anything I find it's better as I don't slip in to the hole of using the second language to explain things rather than finding a way to make my learners understand me in English. My students are better for it, I think, as they have to try and communicate with me, rather than know they can speak their mother-tongue with me.
I am actively trying to learn Italian at the moment, as I plan on settling in Italy. But I won't be using Italian in the classroom, or letting my students know I can speak Italian!
I mainly teach young learners, aged 4-7, from absolute beginner. It is more than possible to control a classroom of young learners even if you cannot fluently speak their language. If you modify your language and make use of mime and visual stimulus then you'd be surprised how quickly young learners pick things up!