There's been some discussion about whether having one language cover a huge area is realistic vs. playability, i.e., do you want PCs to have to spend 20 points on language just to fully function?
Good points. But there's a couple other major factors.
- Humans came to Yrth from another world. There are other types of intelligent beings here. On that basis, I think it's quite reasonable, even in a realistic sense, for humans to "stick together," including having a common language that spreads further than it would have otherwise.
- These societies primarily began at TL 3, not TL 0. They formed a few hundred years ago, not tens of thousands of years ago, so there's bound to be less change. And long distance communication generally becomes more important the higher the TL.