The majority of Inner Sea Races focuses on how the human ethnicities and non-human races of Golarion see themselves, are seen by others, fit into the world, and interact with one another. However, Chapter 4 focuses on the mechanics of building and equipping characters of different races and ethnicities, and contains a wealth of different traits, feats, spells, and equipment for creating a vibrant character immersed in the culture and practices of his or her homeland. It also features a handy heritage appendix, which summarizes the mechanical effects of choosing a particular race for your character in one place.
One of my favorite features in this chapter is a short section written by Creative Director James Jacobs, explaining that a character raised in a region where Taldane (Common) is not the most widely spoken tongue can choose their regional language as their starting language instead of Common. From a mechanical perspective, it makes sense to give all characters a language in common. However, from an in-world perspective, it seems odd that a character born and raised in Wati would grow up speaking Taldane and have to learn Osiriani as a second language, rather than the other way around. So, I'm glad that Inner Sea Races allows players and GMs to choose to represent that Taldane isn't a native tongue for many of the residents of the Inner Sea region.
Whenever the Common tongue is listed as a race's starting language (or among the race's available choices for starting languages or bonus languages), you can substitute for Common any of the other widely used human languages listed on page 6 (for example, an elf who was born and raised in Varisia might choose Varisian as one of the bonus languages she knows due to a high Intelligence score, or a gnome raised in an isolated monastery in Jalmeray might speak Vudrani as his native tongue rather than Common). In these cases, Common always remains an option to choose at a later date if you wish.
In other news: aquatic elf enthusiasts, rejoice! The heritage appendix in Inner Sea Races now presents the stats for aquatic elves in the same format as those for all the other races of the Inner Sea region. You'll be able to find them on page 242!