Yes, but it has not been expressly called 'Welsh' in-universe, as far as I can remember. I've always assumed that in universe, the language was called something else, and could even be slightly different and for the sake of the reader, has been roughly translated into a known language that conveys the oddities without being illegible... Such as Welsh.Â
Kinda like in Star Wars, where Aurebesh/Galactic Basic is basically the english alphabet/english, but called something else.
Calling it 'Welsh' in-universe would make no sense, and also, (more importantly) be extremely lazy and lame. . . . In my opinion.
- P
But hey, as long as I don't have to write the page, I won't refuse more content! Â