Yeah, it's not possible to ask Google to narrow search to Welsh-only documents yet, but if you approximate it (http://www.google.com/search?hl=cy&q=mae+yn+rochester&btnG=Chwiliwch) there are lots of instances of people writing "Rochester" for Rochester.

Today from that map I learned that Caerwynt is the Welsh name of Winchester. It's sort of like having a three-dimensional map, because towns in England either do or don't have Welsh names, and there's generally a reason for it (either they are very old, or they are new but are places that Welsh people have to talk about a lot in modern times), and this forms the third dimension of the map.