"What colour is her hair?"
Translation:Pa liw ydy ei gwallt?
There are people who say bag fi, bag di etc., but like EllisVaughan says, it's extremely colloquial. It's sort of the equivalent of saying "ain't" or a double negative in English - first-language speakers do it, but it's unlikely to be accepted by Duolingo or in polite conversation. First-language speakers tend to say fy mag fi over fy mag - the only difference is that leaving the fi off is more formal. But adding it isn't slang and doesn't give any special meaning. Although you could stress the fi if you want to emphasise it. https://en.wikipedia.org/wiki/Welsh_language#Pronoun_doubling
Not sure about mag fi, but mag could occur - the nasal mutation only occurs after yn and fy, so dropping both pronouns is unlikely to cause confusion. I wouldn't expect to see it written down though, and I don't know how common it is. Never heard of it happening for dy fag di or any other persons, probably because the other mutations are common enough that some confusion could arise. https://en.wikibooks.org/wiki/Welsh/Mutations#Nasal_Mutation