Brittany, or Bretagne in French, sometimes called Lesser Britain in older English texts, is a cultural and administrative region in the north-west of France. It was an independent kingdom and duchy until 1532, when it was conquered by France and incorporated as a province. It struggled to maintain much of its cultural distinctiveness throughout the intervening centuries, with a degree of success. The Breton language (of the Celtic family) is considered endangered, but has been kept alive by various educational initiatives in the region and elsewhere. In 1956, Brittany was designated a French Region.
Brittany in Atlantis
Brittany was the home of François Kersauzon, the man who discovered Atlantis in 1452, and founder of the Atlantean town of Cosquer. When France absorbed Brittany in 1532, the Breton colonies became part of the French empire.
Brittany in In High Places
In an alternate where the Great Black Deaths killed about 80% of Europe's population, the Duchy of Brittany was a sort of vassal state to the rump Kingdom of Versailles in the late 21st century. The people did not speak French, and so it was useful for the people of the Kingdom of Versailles to know some of the Breton tongue.
When Jacques was taken from his world to another one by a group of renegade Crosstimers, he met a man named Dumnorix who was from yet another alternate. Dumnorix spoke a language similar to the Breton Jacques knew, and so the two were able to communicate.
Dumnorix's world was an even lower-tech world than Jacques'. However, no one learned what the break-point of that world was.