So what languages do I have in my DF game?
I've studied linguistics, but what I find fascinating about languages would bore my players, so none of that shows here. It's purely color, and it's surprisingly easy to get good at languages in Felltower. It's mostly grossly simple stuff.
Common: The main language of the area. Not actually called Common but I haven't named it. It surprisingly matches English word for word. Odd, that. Alphabetical writing.
Dwarvish/Dwarven: A defunct language. It's like Yiddish in East Coast US English - it's not a language so much as words that pepper speech. You can learn it and use it, especially to read older dwarven books, but it's not a common means of communication anymore. Might be worth schlepping a Dwarven dictionary with you, but it might not. Only your dwarven grandmother uses this anymore, and whichever thing you call it - Dwarven, or Dwarfish - is the term she doesn't use and it breaks her heart to hear you say it that way. Rune-like writing.
Elder Tongue: An old language, out of current use. Used back in the day by evil wizards, ancient scholars, and still used today by ancient beings, usually of great evil. Not a great way to impress your local clergy, who might report you to the Inquisitors if you walk around showing you know this. Of course some Inquisitors will know it well, to better fight their enemies! Ancient magic books are often written in this language. Wizards sometimes (okay, often) learn this in its written form only, to read forbidden texts and learn dark secrets.
Elvish: A lyrical, beautiful language, with lots of gentle and extended sounds. Used by elves.
Goblin: A fast, liquid language - tonal, like Mandarin. Fluid script, like Thai, Arabic, or Burmese. Used by goblins, largely, and some humans who trade with goblins.
Gnomic: The language of gnomes, this is a very difficult language to follow, deliberately obscure. It's full of ambiguous terms and pithy aphorisms and maxims and sayings, none of which are really clear. It almost seems like gnomes are trying to not communicate with you. Or each other.
Halflinglish: Another "dead" language. Mostly its terms survive in halfling speech in Common much as dwarven terms are used in dwarven speech. Its written form is very obscure and mostly shows up in cookbooks, housekeeping advice books, and halfling mafia coded messages.
Molotovian: The dialect of common spoken in the troll-plagued east.
Orcish: A harsh, guttural language that has more swear words than any two other languages combined. Mostly used by orcs, ogres, trolls, and other folks who socialize with orcs. The written form is crude but workable.
So far, that's it. There are hints of other languages, and some races clearly speak their own, but they haven't been named or described yet. No one knows what the six-fingered beings speak, for example, and no one tried to talk to the lizardmen (who have civilization but lack telepathy, so they must have some kind of speech.) Lots of underdwellers use one or more of the above. Gargoyles often speak common, for example, although it's not clear why. Others use Orcish or Goblin, depending on who they hang out near.