Caught.

Oh, I've caught them.

Student 1 to Student 2 in Korean: "How do you say [horrible bad word] in English?"
Student 2 in English: "[Horrible bad word.]"

The idea is that I hear nothing but radio static from Student 1, when Student 2 suddenly comes out with a profanity in English with me standing right there, which obviously catches my attention. I don't know how long they've been doing this. Obviously, it will still work from time to time because I can't hear Korean as "automatically" as English, meaning if I'm concentrating on saying something in English, the Korean is still likely to buzz right past my ears. But today I caught it anyway.

Clever little bastards.