Who ever said that it was called Welsh in-universe? Did that whole "authors' perspective" fly over your head? Whatever language people are speaking in-universe it is the undeniable truth that the writers are using Welsh for it and/or names that are not necessarily connected to a language per se and thus someone can write a page about how Welsh is used in the story. It'd be a different page than one describing the, say, Morkaeg language (not entirely sure if Morky would be the contemporary name or just an ancient name + probably not the name others have for them, but it'll work for now), which is to say the language of the Welsh-like remnants inhabiting Morcia.
And although I agree it would be the more sensical thing to have the language be its own language and just be presented to the reader as Welsh (although I guess it's left vague to be interpreted by the reader what correlation that exactly means), if we followed Rhoddwr Marwolaeth/Perpetual Doom logic it'd be a pretty acceptable thing to have unexplained references to the Welsh language itself in the story (in fact Perpetual Doom already mentions French, Spanish and probably English).