In the comments to this post by noisms touching on the semantics of titles, user Lich Van Winkle brought up the matter of having varied languages for all manner of creatures. That is a typical practice of mine, and even when I make exceptions for creatures living on a far vaster scale of time and space than humans do, those tend to have exceptions of their own (as I commented there). In thinking on it further during a post-workout shower, I thought my particular approach to handling the languages of giants in my personal setting might be interesting and/or useful to others, hence this post.
D&D draws much inspiration from Greek, Roman, and Norse mythologies, and so I feel no aversion to copying their common element of prehuman civilizations ruled by giants. Since my giants meet my criteria of being a class of creatures living on a far vaster scale of time and space than humans do, there is a common language that they share. Except when they don't.
See, I also like fae (no matter that I shift between "fae" and "fey" with no rhyme or reason), since they're just great at being agents of Chaos that aren't caught up in as many matters of modern cultural moral moors as demons are (nor as many matters of pop culture as goblins or gremlins are, though I treat both of those as types of fae, but I digress). My fae speak variations of sidhe (the low tongue) and sylvan (the high tongue), depending on their standing with respect to faerie courts. However, ogres, trolls, and fomorians (generalized per D&D, as opposed to the actual Fomorians of Irish mythology) fall into both fae and giants, so what to do with them? Well, I took the approach that felt the most verisimilitudinous to me: mash them into a non-standard mixture with some history to explain the muddle.
People who are playing or are planning on playing in my games should either stop reading here or be prepared to feign ignorance of what follows unless their characters learn it in play. I'm used to doing the latter as a GM, so I trust you'll be mature enough to handle it, too.
Ogres and trolls are easy enough to start with. I see them as being brutes that the fomorians cultivated to keep their subjects in line prior to their fall, so they speak a pidgin language mix of the sidhe and giant tongues. From there, it stands to reason that the fomorians spoke a hybrid of the sylvan and giant tongues, hence leading to the divide between the sidhe and sylvan tongues even after the fomorians were cast down. It's a chicken/egg situation as to whether the divide was something the fae took from fomorians or something the fomorians adopted from fae, and resolving it here isn't important.
The fomorian language is more than just that, though; it's also the reason for their present condition of having twisted, malformed bodies. The predecessors of the giants were titans. They had knowledge of what was once known as Godspeak, which present practitioners of mystical arts would refer to as power words, nigh-mythical utterances of primordial provenance. The original fomorian language included pieces of Godspeak; it wasn't enough to dominate the fae by itself, but it was sufficient to be insurmountable when combined with the force of arms of the fomorians themselves and their ogre and troll underlings.
Thus why the faerie courts gathered to work a curse, a sort of reverse-Shibboleth that rendered the fomorians incapable of using Godspeak anymore. When they even so much as brushed against doing so, they would utter words of Chaos instead, warping themselves. Since fomorian magic had been based entirely on Godspeak, losing access to it meant the fae were able to leverage their own magics to bring an end to the rule of the fomorians. Ogres and trolls stayed about in the faerie realm for reasons beyond the purview of this post, as is the interaction of Godspeak with the powers of Law and Chaos.
The fomorians know what they lost, of course, having passed down the legends of it through their generations. Should an enterprising adventurer find a way to break the curse and restore the lost tongue of the fomorians, it would have a great impact on the state of the faerie realm, but none have done that to date.
What of the giants in the mortal realm, then? Well, hill, stone, fire, and frost giants (the dirtbound, to translate the derisive name that "higher" giants have for them) speak dialects of a common giant language, unless one were to compare their languages on a planetary scale. Cloud giants can usually understand it, but they speak a predecessor language, similar to someone speaking Latin as compared to French or Spanish. Storm giants have the same language, save that they also retained some fragmentary knowledge of Godspeak. That's how they work their magic, after all, albeit in different ways to how the fomorians once did.
As for the titans? There's no apparent reason why they wouldn't still know Godspeak, but surely they'd still be in charge of the world if that were the case, right? Unless they were held in bondage, of course, or perhaps they departed to some other realm? The mystery of the titans stands unsolved for now, not unlike the truth of the gods themselves, but that's a story for another time.
For now, let it suffice to say that I made this post for three purposes:
To give an example of something where I think neat categorizations would detract from the essence of the matter (in this case, giants speak a common giant language except for all the ways in which they don't);
To give an example of taking a boring convention (effectively-universal languages) and digging at the places where I could see gaps in the fit as an exercise in worldbuilding; and
To provide fodder for other GMs to steal or twist for their own use.
I challenge GM readers to do something similar in your own games. It doesn't need to be some broad historic lore that could shake up major power structures if the PCs figure out a way to mess with it. It could be something as small as why this cult isn't just a cult, how that potion is different from other potions with similar effects, or what enchantment was laid over a region to change the game mechanics from what they normally are. Minor inconsistencies and quirks are interesting. Don't be afraid of them.