Although no experience in Korea, I have taught in China, where I can communicate in their language. Most foreign teachers in China cannot, but it is really not necessary. I think you are correct that your use of Spanish should be minimal in the classroom. It is a disservice to the students otherwise. I do believe a knowledge of the local language can help a teacher undertand interference problems and also overcome misunderstandings that threaten to delay the class as a whole. A great danger, in my opinion, is that the teacher might seek to use the class as a vehicle for their own foreign language study.
> I teach in Mexico and in my classes I don't hesitate to explain
> things in Spanish. At the university where I work we regularly debate
> on whether or not and how much Spanish we should use in the
> classroom. The general consensus is that a little is OK.

> I am heading to Korea next month, and I won't have the option of
> speaking Korean (at the moment I can't speak any Korean) in English
> classes. I was wondering how many teachers in Asia CAN speak their
> students' language and if those that can use it much in class.