About six years back, I put together a post on the languages of Felltower.
After seeing one of my players put "Reptilian" down as a language on a new character, or alternately taken fluent spoken-only Elder Tongue, I realized I needed to say a bit more.
Language & Culture
In general, races have one language and one culture, making them more like, say, Japanese or Basque than English or Middle Ages French. That is, the people who speak the language are generally culturally connected to that language and possessed of one culture. It's why there might be dialects of orcish but all orcs speak orcish. It's a gross simplification for game purposes, and it's not really realistic or sensible. But as I've said before, the game is about the dungeon and the adventures within. It's not about the nuances of linguistic development, even if that's what I did in grad school.
That said, it's kind of silly to expand it to where "reptile" races all speak one language, since I sure don't make orcs, goblins, halflings, gnomes, dwarves, and elves all speak one language ("Demi-Human?")
Additional Languages & Details
Here are some other languages the PCs have heard about. Generally, these aren't available for purchase for starting characters.
Lizardfolk - A hissing, sibilant-heavy language with connections to smell inherent in its "spoken" form, punctuated by snaps and growls. A non-lizardman cannot learn the language at above Accented (Spoken); even then, it's rarely learned beyond Broken (Spoken.) Lizardfolk has a written form based on pictographs, but it varies significantly by specific group of lizardmen and is only used by their "shaman" - spirit-sensitive spellcasters.
Snakefolk - The actual name of the language is obscure; in Common it's referred to as Snakefolk. Like lizardman, it is hissing and sibilant-heavy. It's believed that different types of snakefolk speak very different languages. The only commonly known form, however, is of the desert-dwelling snakemen of Morthand. There is a written form that closely resembles Arabic script, but with way more snake-like figures.
Dragons - Lots of stories mention dragons speaking . . . but no one specifies what they speak. Presumably, a language like Common, Elven, or Goblinese, otherwise the storyteller would not have understood. Those stories may be apocryphal, too. It's unclear, and even divinations fail to give back a clear answer.
Vegepygmy - "Spoken," this language consists of chest and body slams and thumps, tassel movements, expressions, and timing. There is no written form. To "speak" this language requires Speak with Plants in order to properly convey and understand the intended meaning.
Here is an additional note on Elder Tongue.
Elder Tongue - Can you take spoken Elder Tongue? Technically, yes, but I require an in-game explanation why. Especially if you've learned spoken-only, but can't read it. In most cultures, it's a dead language used primarily in its written form. People don't really speak Elder Tongue. Some ancient, evil beings do, so if you have it at anything above Broken, you must have learned it from one of them. Otherwise you need to use Gift of Tongues. I don't allow people to cast Gift of Tongues and then learn from it; you can't taking Teaching and Gift of Tongues and teach any language you choose. The doubled cost doesn't help, either.