The Universal Translator that is used in Star Trek, as well as its related inventions in other science fiction, is an insane idea if you think about it.
Let’s look at the various ways it can possibly work.
Perhaps it’s simply analysing spoken and written language and interpreting it. This is the most obvious way, but there are no ways to avoid delays during interpretations. For instance, the German sentence Ich kenne diesen kleinen Jungen means “I know this small boy”, but Ich kenne diesen kleinen Jungen nicht means “I don’t know this small boy”; in other words, the second word of the English translation cannot be produced until the last word of the German original has been said. This means that interpreted speech will always be delayed compared to the original, but there’s absolutely no evidence of this in the Star Trek series – people from different planets talk freely without any noticeable delays.
So if that doesn’t work, the alternative is to analyse the speaker’s brainwaves or similar. Apart for the problem that it’s unlikely that different species would have brains organised in similar ways, it would require brainwaves to be present. That is, it would be doable if the speaker was in the same room as you, but how would this work if the speaker was speaking over a radio connexion, as they often do? Do subspace transmissions always carry brainwaves along with the audio? Apart from that, how would this approach work with written materials or audiovisual recordings, given that they don’t have brainwaves? You often see Star Fleet members operating alien spaceships that have all labels written in alien letters, so somehow the UT must be handling this, too.
The second big issue is that it picks up new languages too rapidly. It seems to be enough for it simply to listen to a few sentences in an alien tongue, and it suddenly becomes able to translate everything. If it’s operating on brainwaves, perhaps this would be possible, but if it’s actually analysing the language, this is just impossible. Even if it could quickly establish the meaning of a few words, it would takes days if not weeks before the vast majority of the vocabulary items had been encountered enough times to work out what they meant. In other words, it would have to intersperse its translations with beeps all the time when translating a ‘new’ language.
Finally, there are some occasions where people clearly have learnt an alien language. Why on earth would they, if the UT is so wonderful? And how could they, if they never actually hear the alien languages because the UT interprets everything before the sound reaches their ears? I guess they’d have to switch off the UT for a long time, but that would be such a handicap for a long time that I doubt it would be a frequent occurrence.
Why is there such a deficit of linguistically plausible science fiction?