Place names are bilingual in Wales because cities, towns, villages, hills and rivers are known by different names by speakers of different languages – in our case, Welsh and English. These names have developed over a long period of time because these places have been inhabited or known by speakers of both languages. It is only recently has any official organization given itself the power to change those names.
The existence of multiple names for places is a very common situation throughout the world but somehow appears to be a concept that some people on this island find difficult to comprehend. There are places in mainland Europe that historically have or had three, four or more names. Can’t see the problem, unless you have a thing about people speaking different languages to you.