The Tales of the Yawning Portal has a dungeon called the Hidden Shrine of Tamoachan. In that dungeon there's a language called Olman. It's everywhere, scattered throughout the dungeon. In theory, a character who knows that language could read things and converse with someone of the dungeon residents. However, I'm not seeing how any character can possibly learn that language in 5e DnD. What am I missing here? How come Olman isn't a language that can be actually learned?

Hidden Shrine of Tamoachan is a dungeon from AD&D, the first edition of D&D and as such refers to things that don't appear in 5th edition D&D such as the Olman language. It's from the Tales from the Yawning Portal anthology, that is a collection of previous edition dungeons.

As a note, in terms of in-game lore, Olmec is a dead language, and can only be understood using magic. I don't have my original module at hand, but as I recall from way back then, it wasn't something that had a full script or alphabet, and was never set up to have such or be a "playable" language, even then.

Given it is based on a language family that used a series of highly complex glyphic forms that were not phonemic but structural, what is provided in the game is likely less than 10% of the total possible symbols for things.

So, in terms of a PC learning it, there is no one to teach them the language, as it is only used there, and is no longer written among the peoples that once created it.

Grain of Salt: I haven't even looked at that module since about 81, so my memory may be fuzzy.

Theoretically any PC could learn any language if they really wanted to. Training is something that can be done in downtime. One could use the training rules from the Basic Rules, but they’re kinda unrealistic, so instead I would recommend the rules from Xanathar’s Guide:

 Ten weeks is still a really long time to take off from adventuring though, for something like a language I would allow for a PC to learn at least some of the language during regular adventuring, like while traveling and during rests. Probably a DC 15ish Intelligence check every week with success counting as a workweek studying. Kinda like how Ibn Fadlan (Antonio Banderas’ character) learned the Norseman language while traveling in The 13th Warrior (Eaters of the Dead). Maybe not the written language, but enough of the spoken language to make palaver with the locals. Worth a thought.

As Davyd said, it’s from an old edition so it doesn’t map quite perfectly to this one. If I were writing an adventure like this now, I’d put something like that in for 2 reasons. 1, so the warlock can get some use from Eyes of the Rune Keeper. Or 2, as a plot device to get the PCs to find someone who can translate it. 

Or cast Comprehend Languages.

This something that all too often doesn’t make it into game design and world building - ancient local languages ( hate to say it but I’m not that good about them either) if you take the forgotten realms  the first great culture 35k yrs ago) were the Sarrukh so any of their ruins should have only their language which essentially no one speaks today except for the Yaun Ti with a corrupted/descendant version. Then you have the Batrachi and Aerrie (sp?) , the dragons and Giants, the elves and then you get Netheril with its own language, Imaskar with its own language, Calimshan with its own languages ( think olde, middle and modern English), Mulhorund and other regional languages that players and DMs almost never think to include but that would be used by most folk in a region rather than “common”. Add racial languages and almost anything carved or preserved is going to be in a language the PCs probably don’t know. 

Quote from Wi1dBi11 >>

This something that all too often doesn’t make it into game design and world building - ancient local languages ( hate to say it but I’m not that good about them either) if you take the forgotten realms  the first great culture 35k yrs ago) were the Sarrukh so any of their ruins should have only their language which essentially no one speaks today except for the Yaun Ti with a corrupted/descendant version. Then you have the Batrachi and Aerrie (sp?) , the dragons and Giants, the elves and then you get Netheril with its own language, Imaskar with its own language, Calimshan with its own languages ( think olde, middle and modern English), Mulhorund and other regional languages that players and DMs almost never think to include but that would be used by most folk in a region rather than “common”. Add racial languages and almost anything carved or preserved is going to be in a language the PCs probably don’t know. 

I created a whole Language Tree of all the major languages, cants, and creoles for my world -- but I haven't yet created any alphabets or syllabaries for any of it, and I have only lightly dabbled in the conlang portion. I absolutely dread the thought of trying to come up with a rough family tree of all the FR languages, lol.

It would entail hazard pay, I am sure.

I do have to create the glyphs for the assorted stuff still (right now, as a placeholder, I use different fonts), and I do have a basic conlang dictionary (thank you Vlgarlang) for "Common"., and I have a good idea of the phonemes in use by the different branches.

However, I also have as a general rule a broad degree of illiteracy among the broader population, including adventurers, who get far fewer languages and have to intentionally choose to be literate.

Which will be cool down the road for much the same reason that Gygax used them in the module beign discussed -- flavor and color, with a possibility of some information being able to be obtained through spells.

But I also have an underlying idea that "written word= magic", even though everyone knows it isn't true, that's how they feel about it. 

Given how easily languages diverge even racial languages like orc ought to really be thought of as a racial common tongue  with distinct regional variants  and descendant tongues in use in different areas. A whole lot could be done with language complexity even for a single place. Think of Troy, nearly a dozen distinct levels covering over 1500 years occupation, conquest, migration, etc. do you really think the exact same language was spoken there for the entire time?

Quote from Wi1dBi11 >>

Given how easily languages diverge even racial languages like orc ought to really be thought of as a racial common tongue  with distinct regional variants  and descendant tongues in use in different areas. A whole lot could be done with language complexity even for a single place. Think of Troy, nearly a dozen distinct levels covering over 1500 years occupation, conquest, migration, etc. do you really think the exact same language was spoken there for the entire time?

Well, *diverge* may not have been the best choice next to easily there. 300 to 500 years is not an easy amount of time, imo, lol. it also depends on the way the language incorporates loanwords, and for our purposes any decisions by those with power to enforce certain thinking.

Troy likely only spoke around 5 languages over a 1500 year period, and most of them are going to be from one of three families that have a common ancestor.

I can't speak tot he Orcish of FR, because I haven't looked into the connections between them, but I can note that some languages are more isolative than others, and that is even with the influence of external/neighboring cultural bodies that speak different ones. 

One of the key things that underlies a lot of it is the political power of whatever group has the greatest effective might -- this is in part why English and Spanish are spoken so widely over a huge part of the globe (and you can just as freely add Dutch, French, and Arabic in there under the same basis) -- conquering nations enforce their languages. THe legend of Babel may have been meant to explain the diversity of language to the 12th century scholars that created it, but it also shows a common trope among folks (including me in my early years, lol) to link language to ethnicity.

There is some merit to that, but by and large, even as far back as the 3rd century BCE, it lost meaning if you encountered any of the dynastic or expansion based peoples no matter where they were on the planet. But it also isn't a cut and dried "we conquer you, now you speak our language" because inevitably the conquered language infiltrates the conquering one, and that act changes *both* languages.

When you move through Old, Middle, and MOdern English, alone, you have to start with the understanding that Old English is already a syncretic creole of sorts, and that it has far more base roots from across the channel than ever came from the island itself.

And all of that isn't even counting the phoneme drift issues that come with time.

But I could probably go on about this for hours, lol.  And there are likely five thousand conlang sites out there with inevitably one to five folks who are as deep into it (or likely deeper) than I. I sorta fell away from conlanging except as an incidental around 20 years ago.

I’d think that in a fantasy setting, the number of years that have passed would have a varying impact depending on species. If we stick with 300-500, that’s not even the lifespan of a single elf, while it’s, I don’t know, 15-20 generations of humans, give or take, and even more for some other, shorter lived species. So you’d really need to consider who is speaking the language rather than be able to make a blanket formula for how rapidly it would change.

Quote from Xalthu >>

I’d think that in a fantasy setting, the number of years that have passed would have a varying impact depending on species. If we stick with 300-500, that’s not even the lifespan of a single elf, while it’s, I don’t know, 15-20 generations of humans, give or take, and even more for some other, shorter lived species. So you’d really need to consider who is speaking the language rather than be able to make a blanket formula for how rapidly it would change.

Oh, absolutely! There is a reason that Tolkien's languages were very much static in their nature -- and that's because the memory of elves is so incredibly long.

The same rule would apply here -- and Elves, even gregarious ones, would tend to be more isolative, and far more resistant to loan words as a result -- because loanwords enter through being of value in a certain context, and Elves would have often encountered that context long, long befor ethe modern era. 

THe same would apply for Dwarves, who amp up the isolative aspects even further in terms of D&D -- there is a reason that Balin was able to return to Moria and know what he was doing -- but Gandalf (who was only elf friend, not elf) still had trouble with Dwarf thinking in Elvish language, lol (Speak friend...) -- to use my LotR example for the month.

Dwarves are people who tend to be on their own -- this has the effect of pushing tradition and expanding craftsmanship and creating more rigidity of culture norms, all of which are far more resistant. It also means that they wouldn't use Loanwords -- not just out of time, but because they would make their own words for it rather than borrow someone else's, "because other people don't understand the dwarvish way of thinking and being" as an example.

Now, the really interesting stuff is when you look at Planar beings, who are essentially immune to such things. The language of a planar peoples is more unlikely to have change even a little over a vast period of time than it is to have changed (not impossible, not even improbably, merely unlikely), because for the most part Planar folks of the sort I am talking about (angels, devils, deons, etc) are not affected by mortality issues in the same way that "material folks" are.

Theoretically, one could posit, within s D&D basis, that all the languages of the Material plane are all creoles, cants, and descendants of the Planar languages. I mean, Prometheus brought fire, but Hermes brought language. And then he divided it to piss off Zeus, lol.

Such a thing may not work for my setting (the planar denizens came after humans, and the planes came after them, lol), but for FR, it would be a breeze.

Yes, the languages of “immortal” beings shouldn’t change at all, so that of the various lanar beings is probably unchanged since their creation. For mortal beings I would agree that longevity as well as social structure should have impacts with the long lived species languages evolving more slowly than the shorter lived species. So draconic is probably the slowest evolving language as dragons don’t reach adulthood until 100 years. Elvish probably evolves at about the same rate given “Age. Although elves reach physical maturity at about the same age as humans, the elven understanding of adulthood goes beyond physical growth to encompass worldly experience. An elf typically claims adulthood and an adult name around the age of 100 and can live to be 750 years old.”  So figure roughly a 75 year “generation”. Dwarves are probably next given: “Age. Dwarves mature at the same rate as humans, but they’re considered young until they reach the age of 50. On average, they live about 350 years.”  Half elves don’t really have a language but they might act to somewhat speed up the evolution of elvish while slowing down the evolution of human languages. Humans and most other related species have a generation of around 20 years while groups like orcs and goblinoids it might be as short as 15 years. So given the FR setting we have: 
Dragons - around since roughly -30,000 DR so about 300 generations and even the oldest dragons are only 10-12 generations old so lots time for evolution.
Elves - also around since about -30,000 DR so  about 400 generations.
Dwarves - around for at least the last 14,00 years and possibly the last 30,000 so 280 to 600 generations.
humans - it’s unclear if they are imports ( like the elves) or native but they have apparently been aroun “forever” so call it 35,000 years with 17500 generations.

to see how fast languages can evolve I used English  but much the same result happens with things like Latin. CE 450 marks the effective end of the (western) Roman Empire  so 1573 years since Latin or latinized Brythonic (proto English) were a more or less unified language that means some 79 generations for protoEnglish to evolve into modern English with admixtures from Anglo Saxon Germanic, Danish Germanic and Norman French thru olde English (Chaucer) to Middle English (Shakespeare) to British, American and Australian English. Or for Latin to evolve into French, Spanish, Italian and Portuguese. 

if roughly 80 generations is all it takes to change human languages so significantly then even Draconic of today is almost nothing like the draconic of Dragon Empires of 30k to 25k years ago in the FR  and the Elvish of the pre- Crown wars years is nothing like the Elvish of today - no wonder someone developed the Comprehend Languages spell.