What language is used in JP teams with Korean players?
English? or Japanese? if Japanese how similar is it to Korean?
Japanese, if it's a team in japan, that means that there are at least 3 jp players, so it's usually japanese comms. Sometimes they have translators or do comms in duos such as Munchkin and Medusa, and Astell and Seoldam.
Korean is not at all intelligible from Japanese. Completely different languages, but it's easier to learn the other if you know one, because they have a lot of similar linguistic features. So they usually just learn Japanese.