Historical linguists usually make much of place names, and rightly.  When a given territory changes hands, the spoken language of the former inhabitants may completely give way to that of the newcomers, but the place names normally remain as a perennial monument to the people who first lived there, though they may change to the point where they are practically unrecognizable, like the Celtic or pre-Celtic Eboracum that ultimately became York.

Mario Pei, The Story of Language