Many children in Wales do not even know English until they go to school and learn it, the only time they'll ever speak English is in school lessons depending on where they live, so English is not really a first language in Wales.
Whoever said English is the first language of Wales then they need to read up on the law, both languages have equal parity in the country.
Wiki says otherwise:
English is spoken by almost all people in Southern Wales and is the de facto main language (see Welsh English). However, North wales still retains many largely populated places that speak only the Welsh language, or are learning English today as a second language. Wales is officially bilingual, with 21.7% of the population able to speak Welsh and a larger proportion having some knowledge of the Welsh language according to a 2004 language survey. Today there are very few truly monoglot Welsh speakers, other than small children, but individuals still exist who may be considered less than fluent in English and rarely speak it.
So only a 5th of the Welsh population can speak actually Welsh. Not what you'd call a normal characteristic for a first language really.