He is a dual citizen. If you are a male Korean citizen they have mandatory military service. He could have renounced it at 18, but didn't for some reason.

My question is this: what if the person doesn't speak Korean? Guy shows up for mandatory military service and has never been to the country before and doesn't speak their language, what do they do?

EDIT: This answers my question. Apparently they just have to try and figure what's going on and learn Korean as they go. This dude eventually wrote a book about his experience. https://asiatimes.com/2019/12/military-service-a-must-for-american-koreans/