Welsh is known as the British Tongue, Cambrian, Cambric, or Cymric, and is apparently more English than the language we call English. Our English is a hotch pitch of Germanic, Gaul, Prussian and Latin, the words Welsh and Wales are Germanic, as is the word English.
I know what you mean, I did all the signage on a new school there a few years ago, it was like going back in time. That said though, I was quite surprised that the kids spoke welsh, not just for my benefit, they spoke it all about the place, and slipped Into English just like that. Sadly German...
Also bear in mind that north Walean and South Walean are different languages. Made that mistake once on translations for some signs, put north Walean text on some signs in Maesteg, oops, not popular.
Also, they have it in law that all public signage must have both Welsh and English text.