Not at all. I've got a few friends in South Korea teaching English. None of them know Korean.
Check the left hand side for the forums, there's volumes of information on just about any question you could ask.
Tealit.com is good for Taiwan.
I haven't taught a second language myself, but I am around a few people who teach or have taught languages, and I've picked up on it a bit.
Of course, the people teaching English without knowing any of the local language would claim everything is alright.
But it's undeniable that teaching will be more efficient if you do know some of the local language too.