Wales in Welsh

Someone has put up a rendering of OpenStreetMap in the UK in Welsh. Some of the data is dodgy, but I expect it will quickly be fixed now there is a practical reason to do this. Looks kind of weird having Welsh names for only some English cities (what is the Welsh for Milton Keynes? Heh. But surely there are Welsh names for Rochester and Manchester?)

update: added Welsh names for Plymouth, Bath, Manchester, Colchester, Lancaster, Salisbury. Corrected Tywyn.