Hmm. Even if we acknowledge that the UT is basically tech that serves the needs of plot convenience and agree to suspend disbelief, there still needs to be
some kind of in-story rationalization for it, right?
The closest TOS ever came to an explanation was in "Metamorphosis," actually quoted in the script search CLB linked earlier, when Kirk and Spock were able to use a handheld UT to communicate with Cochrane's "Companion":
KIRK: "There are certain universal ideas and concepts common to all intelligent life. This device instantaneously compares the frequency of brainwave patterns, selects those ideas and concepts it recognizes, and then provides the necessary grammar."
SPOCK: "Then it translates its findings into English." ...
KIRK: "With a voice or the approximation of whatever the creature is on the sending end. Not one hundred percent efficient, but nothing ever is."
The complicating factor here, of course, is that they were using a flashlight-size
handheld UT, and other stories obviously (indeed routinely) relied on something much more discreet and sophisticated. Rationalization: the handheld version (or the equivalent inside a ship's computer) is only necessary to process new languages from completely unfamiliar life forms, like the Companion.
Still, the basic concept confirms what Timo was speculating about. Given what the the UT is observed to do, the only way to explain it is with some kind of telepathic function. We know other Treknology does exist that involves varying kinds of telepathic machine interfaces, even if the in-story handling has been inconsistent (as debated upthread). And the explanation Kirk and Spock give Cochrane validates at least the basics of those speculations.
So let's suppose you don't need the handheld model if you're just communicating with some kind of
known alien language (i.e., anything a UFP computer has encountered before), maybe even that there's some kind of subdermal implant that's useful for those purposes for landing parties. Let's also suppose that it works as Timo speculated by modifying both perception (hearing) and expression (speech) as necessary, thus making it possible to communicate even with natives not equipped with UTs. And let's further suppose, as chrinFinity postulated, that when it's within range it networks with the communicators, the ship's (or shuttle's) computer, and possibly other devices, thereby expanding its data access/computing ability/overall power.
Starfleet officers may need to be trained to think carefully about what they're going to say before they speak in order to get it to operate optimally. But that's a good idea anyway, right?
What problems remain? Well, basically that this seems
way beyond the tech level of most other known Treknology, and also that it raises squicky issues of telepathic
ethics that various episodes have touched on in other ways. But while Deranged Nasat may have been joking with his post about Q-level tech, there's actually something helpful in there. After all, not everything in the Trekverse has to have been invented in the next 300 years. There have been countless high-tech alien races in both the recent and the distant past. It stands to reason that at some point one of them invented a UT that uses a targeted telepathic link to the language and speech centers of the brain, with safeguards to avoid any deeper or more personal areas of thought, and that such an outstandingly useful technology would be passed along to (or pirated by) other civilizations over time as they were encountered (or emerged). IOW, once that genie's out of the bottle, it's not going back in. This kind of UT tech could have been around for millennia for all we know.
So presto! Working collectively, we've provided all the pieces of a solution to the OP's question! The only thing this hypothesis
doesn't really explain is the rudimentary state of UT tech in
Enterprise, as e.g. experienced by Hoshi. But hey, it's ENT. If I need to throw details from that series under the bus in order to maintain continuity for the rest of Trek, I have no problem with that.