Regional languages? Check.
Foreign visitors with odd languages? Check.
Languages changing over time? Check.
High and vulgate versions of some languages? Check.
One thing I don't do enough of, though, is different languages for other races. If you speak Elvish, that covers the elves in the bounded sandbox of my campaign area. I don't put in multiple languages for non-human races, most of the time.
Re-reading Traveller Alien Module 3: Vargr has made that omission clear.
"There are hundreds of Vargr languages [. . .] The most commonly encountered language in the Spinward Marches and adjacent regions is Gvegh. Counting the related languages Gvegh-Aek, Kithnour, Uedhu, and Taeksu, the Gvegh language family is spoke by about 60% of the Vargr encountered in the Marches and neighboring Gvurrdon sector."
The Vargr make the Aslan, with their singular language and culture, seem just a little more alien from them.*
This is a reminder for me for next time - add some variations.
Immortal (or nearly so) Elves could have a unified language of great complexity and historical weight. It could be mastered over centuries of study and use, and while non-elves learn just the vulgate form, the elves learn the full version and are able to convey cultural meanings that others don't live long enough to master. Or they may speak many, many different elven languages, each as complex and nuanced as the next - perhaps by family group, or political or cultural group, or by hobby and interest. They have the time to learn them, so why not? Both fit.
Orcs don't have time to be unifying all of their lanuages - if they're short-lived like I often make them. They're likely to have tribal languages with a lot of overlap in the middle, especially if you use an AD&D-like approach of mutually hostile orc tribes.
Dwarves might have clan tongues, or be that race that has One Correct Way of doing things. Depends on your take - are they insular and family oriented, or feel themselves one very tight, unified culture?
And yeah, to my players, maybe you speak Manticorian, but do you speak Manticorese, it's closely-related but not identical language? Heh.
Something I'll have to make more use of, if/when I run another fantasy game. Or, given the reminder from the Traveller book, a sci-fi game.**
This does get pricey with GURPS 4th edition's language pricing, but that can be addressed with a simple switch - make all languages both written and spoken for the same price, so they're 1/2/3=3 not 1-3/1-3=6 total points to master. Learning a few 2-point languages isn't a big thing, and having enough to get by in emergencies in a lot of languages is pretty cheap. Illiteracy is simply a quirk-level disadvantage for a language, and "broken" assumes illiteracy.
* As an aside, I'm pleased that I still have my copy of Traveller Alien Module 1: Aslan. I knew I'd purchased it, and read it, but I hadn't laid eyes on it in years. I was relieved when I was able to just pull it out of the Traveller pile.
I'm only sorry I don't have all of the Alien Modules.
** I've done it before, but I'm more "hard space opera" and a lot less "fantasy with blasters," and I want vac suits and engines that need overhauls and maybe some weird psi-like powers of limited effect, and generally my players want to run Jedi. It's not a good mix.