Slang's a whole different issue, though. 16th century slang wouldn't be in many instances intelligible.
Here's a good passage that shows how unrecognizable slang and recognizable language aren't mutually exclusive propositions:
Sir Jon Harington said:
There was a very tall and seruicable gentleman, somtime Lieutenant of the ordinance, called M. Iaques Wingfield; who coming one day, either of businesse, or of kindnesse, to visit a great Ladie in the Court' the Ladie bad her Gentlewoman aske, which of the Wingfields it was, he told her Iaques Wingfield: the modest gentlewoman, that was not so well seene in the French, as to know that Iaques, was but Iames in English, was so bashfoole, that to mend the matter (as she thought) she brought her Ladie word, not without blushing, that it was M. Priuie Wingfield; at which, I suppose, the Lady then, I am sure the Gentleman after, as long as he liued, was wont to make great sport.
Now, I can read this just fine, with the exception of actually getting the joke without resorting to contextual reading and thus ruining the punchline.
The joke is that jakes is (was) a very strong word for privy, roughly along the lines of "shit room," I guess. (It's from Harington's
The Metamorphosis of Ajax--get it? Indeed, it's also possible I don't find this particularly humorous because the joke sucks.) But jakes is a 1500s slang term, and the slang is always mutable, and it's humor is often lost over the course of years or decades--forget centuries.
But most of us don't speak exclusively or even predominately in slang; and, besides making the meaning less clear to the contemporary audience (23d century audiences don't pay the bills), predicting slang is usually one of the most arbitrary things an SF writer or a general futurist could spend their energy on. It's unlikely to be correct, and often smacks of trying too hard to world-build.
On the other hand, in
limited amounts, it does serve an immersive purpose ("skinjobs," for example, is great invective; "frak" by contrast is a lame euphemism for an existing expletive, and really depends on the actor to sell it; of course one of those was coined by Blade Runner). It may actually say some unpleasant things about the human condition, however, that the more believable slang terms in SF are racial epithets.
So if the humanity of ST's future is really as multicultural as it's claimed to be, it should have a more heavily hybridized language than it does. Heck, by the 24th century there should be countless Vulcan, Andorian, and Tellarite loan words and grammatical features in Federation Standard.
Fed Standard is kinda one of those fanon concepts, iirc, whereas English as a language is referenced, and as far as we can trust our eyes, is used in signage and is the official language of Starfleet (at least the Roman alphabet is the official script!).
There's also the question of UTs--which work preposterously but assuming it works, we can predict some effects. Universal translation being universally available is likely going to tremendously diminish the value of being polyglot, leading to fewer avenues by which languages cross-pollinate each others' vocabularies.
All the same, yeah, I could see where you're coming from on that more than with human languages. The bulk of them would probably be native animals, foods, and inhuman acts such as the Vulcan neck pinch or Andorian coitus. And, of course, we always like a skosh of new slang.
