"Can I say it in English?" - 영어로 해도 돼요? - 영어로 말해도 괜찮아요/괜찮으세요? (more polite) "i don't understand" - 무슨 말/말씀인지 모르겠어요/못 알아듣겠어요. (말씀 is a respectful version of 말(word, language)) (알아듣다 = grasp, understand, parse. 못 알아듣다 = don't understand) "don't understand" is usually phrased with '-겠어요' to express the sense of "I can't even if I'm trying". It is more polite than using -아/어요 which might sound like casually dismissing what was said.

1. 저 영어로 해도 돼요? 2. 이해 안 돼요/이해 못 해요

I mean "can I say it in english", it's about me, I didn't mean "can YOU say it in English"

당신은 영어를 할 줄 아십니까? 또는 혹시 영어 하실수 있나요? 저는 그 말이 이해가 안됩니다.