Brettonian

Brettonian is the native language of Brettonia, and Brettonian colonies such as Cebeque.
Relatively few non-Brettonians in the Wild West learn Brettonians and most Brettonians that make it West learn a different langu. Though technically, not in the west, Cebeque is a valid narrative setting in Westhammer. Cebeque is still west of the Old World.
By an astonishing coincidence, Brettonian spoken in the Westhammer is near identical to real world Québécois French with a bit of Wild West slang thrown in.