I'm still not completely back to the schedule I used to follow, so there's no reason to wait until Friday to post this link. Linguists studying a pair of little-known languages in northeastern India discovered a third, previously undocumented language, Koro. What's especially fascinating is that the speakers of Koro consider themselves to be a part of the Aka, Aka being one of the two languages the linguists were studying. And culturally they really are clearly connected. The language, though, turned out to be completely different in vocabulary, sounds, and syntactical structure.
While in one sense, trying to work this level of detail into a secondary world fiction would likely just leave readers confused or else bore them with dry data...I think you can hint at this kind of complexity, and it's something I aim for in my stories, the sense that there are always more wrinkles to the diverse societies and micro-societies that make up a culture. When successful, this kind of detail makes the story itself far richer.