I dug out my old Making of Star Trek, to see what Uhura's job description was way back when. It says she was a "highly proficiant communications professional" at age 18. It doesn't elaborate on what "communications professional" entails.
It's then goes on to talk about her "female need for the pleasent rountine of Earthbound home"
It does say she sings ballards from "a dozen different planets" which could
mean she sings in other languages.
I then dug out Inside Star Trek: The Real Story to get the low down on the proto-Uhura, Communications Officer Alden. It says "one of the group's most respected technicians". Lots
of deep info there.
It seems the TOS people never intended or planned for a situation where the universal translator (or Spock's mind meld) didn't work.
IMO, even if not a part of the original plan, giving the communications officer translating and linguistic abilities makes a lot of sense. The UT isn't infallible.