(This has probably been already suggested, but...what hasn't, big ideas-wise?)
So right now, even though technically all the major races (humans, goblins, elves, dwarves...and kobolds? I forgot.) have their own unique languages,
everyone is seemingly a polyglot and can understand everyone in any part of the world, no matter how unlikely it is for them to be speaking a common language.
I understand that the way things are might be better for gameplay purposes, but for immersion+lore+fluff+Fun (maybe?) purposes, a(n arguably) better approach would involve the creation of in-game languages - not necessarily having each of them be different in a "generate unique grammar, syntax etc." way (that would be cool but probably overkill and difficult to get right and not have every word sound like total garbage; it would kill such words and names as the famous Urist for instance, which for those who don't know is 'dagger' in current dwarven), but more on the abstract level, similar to how the newly-implemented culture elements work (music, writing, dances, poetry) - each particular language can be learned like any other in-game skill (under a new category).
Each civilization starts out with their own unique language that all members thereof start out as native speakers of, and can
teach their language to other cultures while learning others' languages, though the actual fluency may affect the way conversations etc. turn out - creating language barriers and other reasons to start fights over. If one knows multiple languages, they can choose which one to use when talking (some preferences could be set in a new menu I'd imagine, mostly ones like "Default language to use in conversations"), which has its own implications.
Over time, languages would die out and evolve in a way similar to civilizations - an isolated dwarven settlement of a specific civilization would, over the course of Armok knows how many years, create their own dialect of whatever tongue their mother civilization used to speak, that would be illegible to newcomers.
Some degree of language family/relationship mechanic could be put in place (which would by extension also allow for dialects and such) not all languages
exist in a vacuum, so even if you are not a native speaker of Western Uristongue, you can still get the idea of what someone using Eastern Uristongue is saying (think Russian vs Ukrainian, or Swedish vs Norwegian) - this would also extent to dialects/variants of languages (American English vs British English for instance, or all the variants, varieties and dialects of Spanish across the world), while trying to understand Cacamespeak would be nigh impossible. (I'm not quite sure on what a good naming convention for languages would be, but it's more likely to be in the style of languages like Afrikaans, Kannada and Cherokee, where the names are nouns rather than
adjectives like English, Spanish or Russian). As such, you would get messages like "You cannot understand the goblin at all." or "You think the dwarf is talking about bandits in a cave." or "Urist is talking about someone being killed in some city.", indicating the degree ofyour character's comprehension. This would of course create problems where neither side can understand each other (and I'm still unsure how communication has historically happened there), so some kind of nonverbal communication system would have to be put in place alongside this (if we want to go for true immersion over 'convenience', which admittedly this entire concept might be
going against, for better or for worse).
Each language could be further broken down into Reader, Listener, Speaker and Writer skills, all of which interact with each other in some way (at least if
we're going off how languages are taught in schools nowadays). This could be further integrated with, say, the poetry/writing system to allow
for people (the player included) to translate works made in one language into others - the quality of which would depend on one's fluency in both the target
language and the original language.
Of course, this is far from a priority and may not even be a great idea to begin with, but it just seems like a very DF thing to implement somewhere down the
road. I probably didn't account for everything in it either (again, language barriers, relationships between languages, and stuff I neglected to even mention
like slabs, of both existing varieties), so feel free to discuss all of this.