How do you say you look beautiful in Swahili?

you look beatiful in Swahili is this- Umkaa mrembo Leo

Almost, but not quite. Umkaa does not have a meaning in Swahili. Maybe the contributor meant umekaa (you have sat or are sitting), but that wouldn't be correct either. Further, the question does not use the Word today (leo).

You look beautiful: unapendeza (you are very pleasing). U mrembo (you are beautiful).

You look beautiful today: unapendeza leo, You are pleasing today (i.e., you please people or you please me). The Word unapendeza carries he idea of more lovely than usual. U rembo leo (you are beautiful today) implies you're not so good-looking on other days. Kupendeza speaks of right now and may refer to how a person is dressed or has done his or her hair. U mrembo means all the time, and anything that limited it today would not be much of a compliment.