[Y_Y]

You pose a great question, perhaps complicated by the fact that pretty much Welsh speakers will also have more-or-less native English (if somewhat Cambricised).

Unfortunately I am merely a Q-Celt and not qualified to comment, though I'd love to see an answer from someone else.

I would venture that if there is a difference it may arise from the relative differences in phoneme classifcation that result from the mother tongue (c.f. linguistic relativity of colour perception). It might even be possible to divine some of those differences by looking at tables of regional accents like those you can find on Wikipedia/Wiktionary, e.g. https://en.wiktionary.org/wiki/Appendix:Welsh_pronunciation

[gizajob]

I’ve sometimes wondered if there are any welsh speakers who don’t speak any English at all. My welsh father didn’t learn English until he was about 8, and his mother’s English was extremely rudimentary when she used to speak to me when I visited as a child. This is in a village near Caernarfon where really nobody speaks English on a day to day basis, everything is done in welsh. Naturally nowadays the younger generation is completely native-level in English, just strange to think about that even 70 years ago there were a lot of British people who couldn’t speak English.