And it would be foolhardy to assume that even that would be able to translate every meaning and idiom faithfully so that there are never any misunderstandings. Many things are impossible to translate accurately between languages, because there are no one-to-one equivalents for them. The best that can be managed is an approximation that compromises or substitutes the meaning.
For instance, subtitlers of Japanese film and TV have a hard time translating shikkari shiro, the default interjection when trying to rouse an unconscious or injured character; it literally translates as "tighten/stabilize yourself," but the closest approximation, "Pull yourself together," seems oddly harsh and judgmental to use on someone unconscious that you're worried about. Some translators just ditch the literal meaning and go for "Wake up!" or "Can you hear me?" or something that fits the context more than the words. A machine translation wouldn't necessarily be able to make that judgment. Then there's the perennial yurusenai, the closest roughly literal translation for which is "I won't forgive you," but it's really somewhere between that and "I won't allow you," so in many contexts a better translation of the sense of it would be "You'll never get away with it!" or "I'll stop you!" At best, even the most accurate machine translation would have quirks like that, phrasings that don't really sound right in context. Or else they'd only be approximations that lose some of the speaker's intended meaning, which could lead to miscommunications of a sort you never see in fiction where UTs are used.
Not to mention that real-time translation couldn't work, because not every language uses the same word order. A lot of Japanese sentence structures are the inverse of English. For instance, instead of "I'm Christopher, a 26-year-veteran science fiction writer who has a story collection coming out soon," it would be more like, oh, say, "A 26-year veteran writer of science fiction who soon has a story collection coming out, Christopher I am." So you'd have to wait for the speaker to finish the sentence before you could get an accurate translation. The kind of instantaneous translation you see in fiction is a simplification for dramatic convenience. It would never actually let you, say, fool pre-contact aliens into believing you were speaking their language fluently. (I love the way Star Trek Beyond handled machine translation, practically the only time Trek has ever depicted it realistically. Although I think there was a scene in Discovery season 1 with Burnham on a Klingon bridge that did it similarly.)