The answer to this question is tied to your intended audience and your character of perspective. Let's start with audience: If you're writing for an English-speaking audience, then the book, including the dialogue of Korean-speaking characters, should be written in English. You might establish that the characters are speaking Korean in the narrative, if this isn't already clearly implied. This is particularly important if you have a character to switches between languages or if the use of a particular language has significance to the circumstance.

The other issue to consider is that of perspective. If a moment is revealed through the perspective of a character who speaks only English, then you are bound by that limitation. He may hear the conversation and it should be rendered in the way that he experiences it. He won't know exactly what is being said and he won't know the individual lines of dialogue that make up the conversation. Instead, the narrative can focus on tone, facial expression, or gesture. The character may even make conclusions based on this information.

The reality of multiple languages can be an interesting layer to your novel. Use the characters' relationships with the languages to reveal additional insights. In Yiyun Li's short story “A Thousand Years of Good Prayers," Mr. Shi visits his recently divorced daughter in America. During his visit, he befriends a woman from Iran and though they both “speak little English" they meet to talk regularly. At one point, Mr. Shi wants to tell the woman about his desire for the time when his daughter was younger, but this proves too complicated: “But he is sure, even before he starts, that his English would fail him. Besides, it is never his habit to talk about the past." This offers interesting insight about Mr. Shi's character.