[RPG] If a skeleton learns a language, can it speak it

Yes, its possible to learn and speak druidic without being a druid.

The Faiths & Philosophies player companion introduced the Druidic Decoder feat, that allows a character to decode and even learn druidic without taking levels of druid.

This means that the language isn't magically protected from means to learn and speak it by non-druids, they merely don't share it's secrets around, nor publish any dictionaries that can be found on the local library.

Keep in mind that you do not choose languages when you cast Tongues, you simply grant the ability to speak any language of any intelligent creatures. The spell doesn't grant the knowledge of that language, it simply makes communication possible. Similarly, the spell Comprehend Languages will allow you to understand what is written on a druidic text, but not give you the knowledge to be able to translate it to common.

So, this player character knows Aquan, and someone wants to speak Auran to them.

As Aquan and Auran are both dialects of Primordial, that means they are mutually intelligible. That is, someone who knows Aquan can understand Auran, and someone who knows Auran can understand Aquan. The PC can talk in Aquan, and the NPC can talk in Auran, and both can understand one another.

It is not possible, according to the book, for someone to understand Aquan but not understand Auran. The languages are just too similar to one another for that to be plausible. It’s like someone speaking African-American Vernacular English to someone speaking Indian English—the languages sound pretty different, and they use some different structures and grammar, but they’re close enough that it’s hard to imagine that these two wouldn’t be able to understand one another. Have a try at reading the Scots-language Wikipedia article on the Scots language, for example: you probably can get through it, because Scots and English are mutually intelligible. Auran and Aquan are at least that close, according to the books.

This is in contrast to other “dialects,” such as those of Chinese, which are not mutually intelligible: someone who speaks Cantonese has no particular understanding of Mandarin, and vice versa.

So no, the player cannot choose to know only Aquan; there’s no such thing as knowing that without understanding the other dialects. Moreover, the language choice option covers the entire language family—that is, they don’t just understand all of the Primordial dialects, but they can speak and write in them too. To choose not to have that aptitude would be to diminish the value of the language choice, which obviously isn’t the end of the world, but as a DM I’d be somewhat leery of it, at least in a game where I expected languages to matter in the first place, and in any event the official rules don’t really allow it.

But more importantly, allowing the player to only really “know” Aquan (and therefore be able to understand the other dialects but perhaps not be able to speak or write them) is simply unnecessary. Nothing stops the player from freely choosing how to characterize this choice for their character: they can easily say, sure, my character knows Auran and Ignan and Terran and Primordial too, but they speak all of those with an Aquan accent, and will sometimes use Aquan-specific idiom and slang inappropriately in those languages, because Aquan is really their languages. The advantage of this, over saying they only know Aquan, is that if it ever comes up, the character can get by in those languages—which is usually a good thing, because “oops, no one knows that language” tends to be a bit of a buzzkill interruption in the game more than something interesting (and in a game where it was interesting, I’d be more concerned about the self-nerf since apparently languages matter here).

Characters of races or what have you that specify they know only Aquan or whatever would have a lesser degree of understanding, and would probably struggle mightily to express themselves in any of the other dialects.

But in both cases (only “knowing” Aquan vs. knowing Primordial but with an Aquan characterization), there may be nuances that get missed, cultural details that aren’t apparent from the mere meaning of words, and so on, but that has as much to do with the cultures each is coming from as it is to do with the actual language used. They also might have to speak more slowly and carefully, and possibly dumb down their language to avoid dialect-specific or culture-specific idiom or grammatical structures. Personally, I would not typically want to get mechanics involved in this—disadvantage on all checks seems much too harsh to me, for example—but keep this a matter of roleplaying.