You really believe that everyone in Starfleet learns English?
As I said, it would be unfeasible to expect a crew in crisis situations to rely entirely on slow, imperfect machine translation that might break down while the ship is under attack or under the influence of some weird space phenomenon. Military officers would naturally be trained to do their jobs without complete dependence on technology, and that obviously means that they would learn
some common language. This should go without saying. It just happens to be English in the case of
Star Trek, because we've been told outright in explicit dialogue on multiple occasions that it is English. It astonishes me that this is even a debate.
I... emphatically do not interpret the universe that way. :-) I could certainly buy that- say, in the case of Metamorphosis- everyone present was speaking English. But the idea that all Starfleet officers do seems unnecessary (given the existence of the UT; certainly, yes, as a redundancy in case it fails, but Trek famously doesn't consider real-world redundancies like that), and needlessly culturally-imperialistic, for the future Star Trek is portraying.
"Unnecessary?" What a strange thing to say. Most people on the planet Earth today speak more than one language. That is the norm, not the exception. Throughout history, it has always been normal for people in multicultural societies to speak at least two languages, their own local language and the shared common tongue that allows mutual comprehension with people from other communities. For instance, Jesus Christ spoke Aramaic as his native language and Greek as the
lingua franca of the Eastern Roman Empire, which is why the New Testament was written in Greek, for the comprehension of as many people as possible. ("Jesus Christ" is itself a Greek translation of "Yeshua the Messiah.")
In the present day, English is the accepted international language of space travel, science, engineering, and commerce. Since
Star Trek depicts a future where human interstellar civilization grows out of 20th- and 21st-century spaceflight, it follows logically that English would remain the common language of Starfleet, as we have been explicitly told that it is. That does
not mean, as you somehow seem to be assuming, that other languages would cease to exist or that people would
exclusively speak English. As I've been trying to tell you, that is not how common languages work. A common language is a way for people who speak different languages to understand each other without having to rely on the imperfections and delays of translation (human- or machine-mediated).
Especially when we have seen the UT applied in situations like The 37s and Little Green Men to apparently be the default, always-on system by which communication is managed in the 24th century. (The 23rd, I would entertain debate on...)
Once again, you're conflating two entirely different subjects, whether Starfleet personnel would learn English to communicate
among themselves and whether they would use translators to communicate
with others. Obviously translators do have their uses, in cases of interaction with people who don't already know English. It makes sense that people on away missions would have their translators turned on by default so that outsiders could understand them. But as I've explained, it would be absolutely idiotic for Starfleet personnel to depend on machine translation as a matter of routine communication with their own crewmates aboard ship.
It's like transporters -- it makes sense to use them to travel from the ship to a planet surface or another ship, but it would make no sense to build a ship where there were no doors or corridors and transporters were the only way to get from room to room. The fact that a technology is useful in
some contexts does not require it to be used in
every context, especially contexts where the crew would be helpless if the technology they were dependent on were to fail, as technology on starships frequently does in crisis situations. Technology exists to supplement a crew's own abilities, not to do basic jobs for them and leave them weak and helpless.
I always found "Little Green Men" ridiculous in claiming that the Ferengi characters on DS9 relied on translators by default, rather than learning the dominant language of the Starfleet personnel who'd been running the station for years. As I said, the fictional conceit that machine translation is instantaneous and absolutely perfect makes no sense if you apply any informed thought to it, because language just does not work that way. No machine translation would ever be as quick or reliable as actually speaking the language. I can accept a work of fiction glossing over the flaws in machine translation as a dramatic convenience, but claiming that those flaws do not exist and it's actually preferable to rely on machine translation rather than learning another language is just not something I can suspend disbelief about.