I have noticed that when alien races and their languages are mentioned, it goes like this. Klingons speak Klingon. Romulans speak Romulan. Vulcans speak Vulcan. On Earth we have a few thousand languages, so why has Star Trek never looked more into the languages of other worlds? Surely, they have as many languages as we do, maybe more. A passing mention would even be in order, right?
because the vast majority of the time, we see the aliens in a particular situation. In the same way, we see starfleet officers, and they speak English. Of course, that doesn't mean in China Humans aren't speaking Chinese. And in the same way, just because we see Klingons speaking Klingon, doesn't mean in some country on their homeworld they aren't speaking a diofferent language. Anyway, how do you know you haven't heard five different Klingon languages? How many different ways have we heard Klingons call for transport? And also, given the universal translator, how do you know that they haven't all been speaking lots of different languages, and they've all just been conveniently translated into English?
We may have, but we also have a small handful of languages that are easily the most prominent. By the time we become a unified Earth government and make our way into the galaxy, it's probable that we will adopt a single primary language (probably English or Chinese). Alien races probably have had a similar history.
From Broken Bow: HOSHI: What do you know about these Klingons? ARCHER: Not much. An empire of warriors with eighty poly-guttural dialects constructed on an adaptive syntax. The Vulcan spoken by T'Pol, spoken by Spock (and the Amok Time wedding party) and spoken by Tuvok might have all been different languages from each other, with the exception of a very few common words and terms, like Pon Farr. The Vulcan Priestess at the beginning of TMP might have been speaking a forth language, perhaps a ancient language like Latin.
I always sort of assumed "dialect" basically meant what we would understand as "language." I mean, English and Japanese are likely to be far more similar to each other than English or Japanese is to Klingon Dialect No. 46. (They may even be real cousins, albeit distant, if human language has an unbroken lineage. The Japanese people are real cousins to Anglo-Saxons, after all.) I've also supposed that some or many alien species have constructed languages, ala Esperanto, specifically designed for realtime translation and thus interaction both between different ethnic groups and between other species. I'm no linguist, so I don't know if some languages might be more amenable to realtime translation or not; I used to think so, but now I'm not sure. Romulan, being the product of a relatively small and perhaps ethnically homogenous founder population that definitely had recording technology, really could be just one language. Like say if there was a one-world government coming and ten million Americans said "F it" and took off for Alpha Centauri, we could reasonably expect them to speak English, even when there were the far-flung billions of them two thousand years later.
I assumed with all the thees and thous, that T'Pau was speaking in an ancient language mostly used in rituals on "modern" Vulcan. (think Latin)
It's all about context. Most aliens probably generalize English as "Human" like humans generalize the most prominently used Klingon dialect as "Klingon."
I imagine in a few of the most repressive societies, other languages and dialects were actively suppressed. I tend to think this could be the case on Cardassia, though we never heard their language.
^^ It isn't like English is the only Earth language ever spoken, Uhura speaks Swahili, Chekov swore in Russian and Scotty swore in Gaelic. And Picard occasionally butchered the French language.
I don't think we near enough alien languages to assume any one represents the entire species. And it's fair to assume that once a species gets to the point of being able to quickly travel and communicate with people across their entire planet, that more dominant languages will emerge. Even now English is becoming very widespread across many countries as a second language. I can see this being a natural progression for many of the species in Star Trek.
The alien societies in Star Trek also always only have one government, one clear representative that answers the calls speaking for everyone, etc... even the first contact worlds.