User blog comment:Gamer124/Races of Nirn and Cultures of Earth Similarities/@comment-4976111-20130216144134

Bretons are likely based on their similarly-named real-world counterparts. They have some French names, but because of their real-world proximity to Britain, you could go further and say that they're based on the Medieval West-Europe knights that once inhabited the area. During the era of knights, there was a point where the cultures of France and England met. Due to the Bretons of TES having some French and slightly Medieval names, I would guess this.

Something else interesting to note is that nearby the province of Brittany in France is Normandy, which was inhabited by Vikings who mixed with the natives. I'd see this similarly to how Skyrim is located close to High Rock. But hey, it's just a theory :P