Well, if you're using the default Pathfinder setting of Golarion and the Adventure Path material, they focus a lot on "human" race and culture. Golarion is a deliberately humanocentric world - elves and dwarves tend to keep to themselves and halflings are often just a slave race. There's a host of human ethnicities (Chelaxian, Keleshite, Kellid, Vudrani, Varisian, and many more). Check out the Human entry at PathfinderWiki, there's 23 human ethnicities and that's not counting the many Asian and African sub-ethnicities. They have published supplements for many of these and/or the different countries that go into depth on society and culture of the different parts of the continent. There's trait and feat and equipment choices tied to the races and cultures.
For example, I'm playing a Vudran woman who's a grad student at the University of Lepidstadt in our current Carrion Crown campaign; even for a fringe ethnicity not covered by an explicit splatbook yet I can choose traits including Latent Psion, Open Palm of Irori, and Voice of Velvet. Tied to my home land of Jalmeray I can pick Candidate for Perfection or Secret of the Impossible Kingdom. Searching on "Vudra" in Hero Lab I see a variety of magical items that are specifically Vudran... Even outside the direct rules support, so far my experience has been pleasingly ethnic, I'm in a land of Transylvanians and they don't know what to make of me, but I've actually run across two other Vudran women in the AP so far and it's been a nice bonding experience (plus I get to make korma).
So Golarion has immense fluff support for ethnicities and cultures already, ties those into the adventure paths, and also has trait and other rules support. It doesn't make them equivalent to races in terms of actual game bonuses, but real human ethnicities aren't 20% stronger than others or have low-light vision... But it's enough mechanic for what it's trying to do. We've had Mwangi, Chelaxian, Tien, etc. characters and they've been very distinctive.