Let's not forget that the human brain is already a Universal Translator. It takes gibberish for input and creates interpretations for output. Sometimes those happen to be correct interpretations; most often not. But the brain's one forte is self-deception: garbage in, wonderful truths out is what it was built for, and it achieves that by smoothing out the edges.
Terhe aer pelnyt of cloo emplaxse of tihs. Say, it didn't take your brain half a second of extra time to ignore the incorrect ordering of letters in that sentence; ignorance is the real human superpower.
The UT could easily exploit that. It wouldn't take much tickling of the brain to make us see the opponent's lips move in synch with what our ears are telling us (hell, we can do that without
implanted brain-ticklers today!), nor to make us hear perfectly understandable phrases when alien speech hits our ears, an implant translates about 75% of it very coarsely on the way to the brain, and finally the brain does its usual ignoring on the input and spits out some understanding.
As for the translating of new languages vs. old ones, VOY "Basics" is a cool example of this actually making technological sense. As long as our heroes wear their commbadges, which connect them to the vast computing power of the starship, they can easily talk in English, Talaxian and Kazon alike. Once the badges are taken off them, they can still
converse in all these languages, though (and no, we cannot really assume that Neelix would have learned perfect English, or that the random Kazon extras would have, or that our heroes would have mastered the alien languages - heck, for all we know, even Tuvok only speaks Vulcan, and relies on the UT to communicate with his comrades!). BUT without their badges, they cannot learn a new language, and cannot communicate with the local cavemen!
That logically follows if humans (like Ferengi) have implanted UTs that contain known languages but lack the computing power to translate unknown ones...
Or that T'Pol is imposing the Vulcan word for the new species onto Hoshi
Indeed. Just imagine how the scene would play out in slightly different circumstances:
Communications Officer T'Pol: "The giant unidentified ship is hailing us now. They identify themselves as the Nutsy Shterrnenrroich. Do you know anything about these people, Captain Archer?"
Archer (shivering): "Nazi. That's pronounced Nazi."
The trick here being, no actual Nazi would ever pronounce the word like the English do. In German, it's not Nooozzzy, it's Nutsy. Obviously, Romulans and Vulcans might have completely different ideas about how to pronounce the name of the former culture - and Vulcans would insist on their version, just like we'd insist on Nazi over Nutsy (despite the comical potential). We wouldn't dignify the name with its real, native pronunciation, and neither would T'Pol.