1. It’s not language remaining intelligible but not changing AT ALL. That by itself is more fanciful than warp speed and the transporters combined.
That's a very small bit of poetic license to forgive. Realistically, it's entirely possible that the language would still be intelligible, even if there are more differences in detail than the fiction chooses to depict. It's close enough that I have no trouble suspending my disbelief.
Fiction is not meant to be taken absolutely literally. It always glosses over the details. Do you get upset that characters in sitcoms don't lock their front doors so their neighbors barge in unannounced, allowing scenes to progress without dragging to a halt for the business of letting someone in? Or that cars in driving scenes often don't have their rear-view mirrors so that the camera can see the actors' faces through the front windshield? The details are always negotiable for dramatic convenience, but that doesn't invalidate the broad strokes of the story being told.
2. Advanced communication technology is the least fanciful future technology in Star Trek. Again, we’ll have real time translation software THIS DECADE.
I've already explained why "real-time" translation is impossible. Different languages have different word orders, so you often don't know what a sentence is really saying until it's done. Trying to translate every word immediately would result in frequent errors and bizarre sentence construction.
And again, translation is never perfect. Not every concept or idiom is translatable. Even the best machine translation between two languages is never going to be as flawless as fiction pretends, and is never going to be preferable to people actually learning and speaking in each other's languages.
Also, there was no creative purpose to making contemporary English the lingua fanca of the galaxy. It wasn’t a statement about linguistics, history, or philosophy, and as a viewer I was grossed out by both the jingoism of it, of the English language practically goose-stepping across the stars, and the young-eating delusion of it being our version of it doing so.
It is simply a fact of history that English is
already the lingua franca of aerospace, science, commerce, etc., as well as being the most widely spoken second language on Earth. Assuming, as Trek does, that humans begin to colonize space in earnest in the near future, while current cultural conditions still prevail, it's pretty much inevitable that the first generation or two of space settlers would use English as their primary tongue, at least for communicating outside their home communities, which is what a lingua franca is for. There's no reason to assume it would be forced on anyone, because it's already the default.
More to the point, as I've been saying, fiction is written for the benefit of its audience. In Japanese shows, space travelers and aliens all speak fluent Japanese, and alien invaders always target Tokyo instead of Washington or Beijing. In British shows, aliens all speak English with British accents. And so on. It's not jingoism, it's just writing for your audience.