I have often translated these sorts of artists as printed: “Tradicional” instead of [traditional] or “Anonyme” instead of [anonymous] seems fine to me. Sometimes I’ll use “French Traditional” or whatever as credit as well if that’s what’s printed, etc.
I don’t much like the idea of making up a translation that is not printed though. In that case I’d personally probably just default to the, well, default - except maybe in a pseudo-release