Responses
Since I'm not sure how far you're in:
Joon is speaking in Japanese for a lot of the first season to Daiki and his staff, with the occasional Korean muttering when he's talking or thinking to himself. It is revealed later that Daiki is some generation of Zainichi, hence his ability to understand Korean. Toma and Daiki's staff do not understand Korean and Joon exclusively speaks to them in Japanese, since he is fluent in both.
When two culture met. How come they understand korean and japanese when they talk to each other