"I don't speak Korean" : 저는 한국어 할 줄 모릅니다. / 저는 한국어 못 합니다. "Sorry, I didn't understand that" : 미안해요. 이해하지 못했어요. / 미안해요. 무슨 말인지 모르겠어요.

감사합니다!

In general, Koreans say "네? 무슨 말이에요? (Nae? Mu-seun-mal-e-ae-yo?) = Sorry? what do you mean? when they don't understand what somebody says. It implies like I didn't understand what you said. Can you say that again?