To say bon appetit in Korean, it would be 맛있게 모고 ("Mashike mogo") That was one of the first Korean phrases I learned, but I also found a YouTube video showing you how to pronounced it :) https://www.youtube.com/watch?v=_mSy4A0cYEQ

'Eat well / Have a good meal.' => 맛있게 먹어 / 많이 먹어. - 맛있게: adverb of 맛있다(delicious). means "enjoying it", "with gusto/relish", etc. - 먹어: imperative of 먹다(eat), so it is just "eat!". - 많이: literally "in good quantity; a lot", but really means "heartily, as much as you like".