I know it's not a literal translation, but doesn't English usually express this idea more naturally as "I've never been there"?
Yes. This tense of "be" doesn't exist in English, or at least, it's indistinguishable from the imperfect. So as you say, that's the closest most natural way of expressing it in English.
Wouldn't it be usual (or perhaps less colloquial?) to omit "ddim" in this sentence?
You can omit ddim in sentences like this as long as you put the erioed in its place.
Fues i ddim yn yr Alban erioed or Fues i erioed yn yr Alban (I was never / have never been to Scotland)
Fuon nhw ddim yn y wlad erioed or Fuon nhw erioed yn y wlad (They were never / have never been in the country)
Thanks. It's good to get a Welsh-speaker's view. Would it make a difference if you put "erioed" before "yna", if that is possible?
I'm not sure actually, I certainly wouldn't bat an eyelid at it but I guess there might be a reason for it not to work like that.
Ok. Diolch. Better leave it that then! If I could get to Wales, I would enjoy listening for how different people say this sort of sentence. But I'm stuck in France for the duration, as we used to say during the war -- clue to my age!!