Apparently the editors of this course think "'holidays' would be unusual in English in this sentence" (so says the little note correcting my "error").
It sounds perfectly fine to me. I'm a native speaker of (Canadian) English. I keep getting caught by the fact that the course doesn't think "go on holidays" is good English.
It's possible I just have an idiosyncratic personal grammar here, I suppose.
Thanks for the post and the explanation. We're happy to add alternative forms for the Welsh to English translations, we weren't aware of this as a valid dialectical form, now that we are we'll attempt to add it.