With no other context, 'No, I haven't gone yet' sounded so odd in my head that I accidentally switched it to 'No, I haven't left yet' despite actually remembering the meaning of mynd. Upon pausing to think, I've decided the reason is I wouldn't use 'gone' in such a sentence UNLESS I included where I haven't gone to, such as 'I haven't gone to the store yet'. If I don't include the place, it's only because it's already been mentioned: 'Have you been to the fair? No, I haven't gone yet.'
Which makes me wonder about the Welsh; is this 'I haven't gone yet' in the sense of 'I haven't left yet', in which case my 'mistake' is probably the preferred translation though not the most literal, or is it 'gone' in the sense of 'I haven't been there yet', in which case 'left' obviously wouldn't work as a substitute for gone. Or does it mean both in a way that doesn't quite work in English? In which case 'left' or 'been' would both work but 'gone' would still sound odd in English.
In the UK, "No, I haven't gone yet" could mean either "left" or "been" and wouldn't sound odd at all.
Thanks for the info, I find it interesting how English differs around the world. And it definitely makes sense that a Welsh course would favor British English considering the likely source of native speakers. So that probably means it works both ways in Welsh too, as been or left (or just gone :) ). Thanks for helping to clear up my confusion surrounding the word 'gone'.