More interesting though are mistakes that come about because they drag into their English something from their first language ... like where they put the verb, how they negate things, the prepositions they use ... it's all very instructive.
But ...but ... you don't get that when native Welsh speakers from Wales talk in English ... because, overwhelmingly they will know English as well as they know Welsh, or maybe better.
So one thought is ... if you could find a Welsh speaker who only had a poor grasp of English, maybe a Patagonian with Welsh as their first language, what mistakes would they make ?
I suspect they'd tend to put the verb first quite a bit, they might use Welsh ways of making the possessive, they might even have a go at mutating some adjectives!
So, as an experiment, it would be fascinating to listen to such an individual speaking English to see if any of this happened. Maybe I could find some on Youtube.
A second, and perhaps more accessible experiment, would be to find Welsh Welsh speakers who have some knowledge of, say, French, Spanish or German .... I strongly suspect that when they spoke in French etc they they would do all of those things. I think I might well have a go at that ... again, youtube might be a source, if I can think of a suitable form of words for the search!
That's enough thinking for one day .. here's another rather spiky song from I Fight Lions ... "Chwara' hi'n saff" ... off you go lads ...