Well, the handling of languages in ENT is actually one of the more believable concepts, even though it still takes some suspension of disbelief. My theory about Hoshi has several components:
1. Hoshi is a genius. We see as much in "Observer Effect" and when she decodes the Xindi weapons codes. As a numbers and crypto whizz, she's also a programming genius by default, which would explain the rapid progress of the UT. But even after a year (at the time of "Precious Cargo") the UT still needed some time to riddle out an alien language. I suppose they just left out this "adapting phase" in later episodes as it simply didn't add anything new. We'd seen it already.
2. My suspicion is, that Hoshi also has latent telepathic abilities. That would go a long way to explain Hoshi's ability to riddle out alien languages.
3. We still look at ENT from today's technological perspective. The UT might only be a small device, but for all we know, it could beat today's Super Computers my magnitudes. Think about it. Today's mobile phones have more processing power than PC's of 20 years ago. And the time of ENT is still 140 years away.
4. When you learn foreign languages, the more you learn, the easier it gets. If you know Russian for instance, you unterstand Polish, Ukrainian, Bulgarian, Serbian and other slavic languages fairly well. Of course not everything, but you get the gist of most of what is said.
If we are to be believe TNG, all humanoid races developed from a proto-humanoid race, so there are connections even between wildly different languages.
All in all I think that the language concept in ENT was handled fairly believable.