How to Get a University Job in South Korea: The English Teaching Job of Your Dreams
"Use Correct “Language” At your interview it is especially important that you realize your position of inferiority to your interviewers. If you speak any Korean, you should use the formal verb endings (“imnida” instead of the “yo” or no ending). You should use a 2-handed handshake, complete with head-bow and not make too much direct eye-contact during the greeting and introduction phase of the interview."
The general principal of this applies to whether you speak Korean or English in your interview because you always need to remember that Korea is a very hierarchical culture and that you, the interviewee are way, way lower in the pecking order than your interviewers. You should always use formal language and if you speak Korean, be extremely careful to use correct verb endings. Remember: it's far easier to offend someone in Korean with inappropriate "politeness" than it is in English. While your interviewers may have some sympathy for your weak attempts at Korean, if you make a mistake and address them as someone in a lower position than you, you will be in big trouble.