While it makes sense from a production standpoint, the way they show communication with alien species using the universal translator makes no sense in universe because the mouth movements are obviously English.
As a tv show, it is much more time and cost effective to just have the actors speak English instead of having them do random mouth movements that will be dubbed over in post. Plus it would look weird to viewers.
But thinking about it in universe, the universal translator would be like watching a live action movie dubbed in real time. If you're a Star Fleet officer speaking with a Klingon who is speaking Klingon Hol using the universal translator, you will hear English but still see the Klingon's mouth speaking Klingon Hol. So when you hear "What do you want?" you still only see their mouth move enough to say "nuqneH".
I understand why they don't do it from a production standpoint, but it would be cool to see what communication with aliens would really be like. Plus you could really see who actually knows English vs who doesn't.