That would depend on how isolated their communities are and-or how much regular interaction there is between them.
I mean, in the real world we have significant dialectical differences within the same country in theory speaking the same language in at least two ovbious instances - the UK and the USA - and that's with lots of interaction between them.
And they're all the same language. I
did say there would be regional differences, but they would be more like UK versus US, which, while there might be very broad differences in places, it's not like the differences between two different Romance languages, and especially not like the difference between English and Cantonese.
Not in my games. Literacy is only guaranteed for mages, as it's a job requirement. Everyone else, if they want a shot at literacy, have to pull out the dice during char-gen. If illiterate, a character can spend several in-game months learning literacy (in one language!) once the campaign begins.
But your games are not D&D standard. And that's fine, I'm not saying that your games have to be. But your rules on language are neither RAW nor RAI and thus are not typical for D&D games.
Eberron actually has public education, for instance. There is, or at least was, lots of private education throughout Ravenloft. The Realms has a god of writing and I'm sure his clerics teach literacy.
I'm thinking at the least it would be more like a Texan trying to converse with a Highland Scot.
At the beginning, sure. But if that beginning was any longer ago than just a few generations, growing differences in laugnage between different communities (and, later, cultures) will soon appear.
Perhaps. I think you're underestimating just how much language evolves over time.
I don't think I am. Language
does evolve over time... in the real world, where people have short lifespans, literacy was uncommon to very rare, and transportation, and thus the spread of language, was highly limited. But in D&D world, people
don't have short lifespans, literacy is very common, and transportation includes teleportation, magical communication, and flying mounts.
Sure, there's a huge amount of difference between modern English and, say, Old English--but Old English was spoken about 1,500 years ago. In a D&D world, there are going to be creatures that are
still alive after 1,500 years. That's only a handful of elf generations (and if you use the idea that elves reincarnate and remember their past lives, then even
that doesn't matter). That's
one generation of lich or vampire. s, relatively few deal with the multuple tens or hundreds of thousands of years that the real world had to deal with.
With humans, in the real world, it seems to be that the first small language developed at point X in Africa and then people spread out, developing their individual languages as they went. If, as I mentioned, the gods created (modern) humans fully-formed and plunked them down all over the place knowing the same fully-developed language, then the changes afterwards are going to be much smaller.
Plus, let's face it--nearly all D&D settings are static. New technologies and philosophies just don't take over. There's no reason language would also change that much.
For the record, I'm not saying that D&D games should only have one or a very few languages. I'm saying that it's not actually illogical for such a setting to have few languages.