That's a good question. There are quite a few schools in Korea like this. There's one nearby called the Foreign Language High School and all the classes are conducted in their chosen language of study. Since English(or whatever language they are studying) is secondary to the subject they are studying (ie. math, history, science) I would guess that falls outside the realm of teaching ESL or EFL. Most of the teachers who work at these schools are certified teachers from their native country, not just TEFL or TESL certified. That would not be enough.