Ah, the big question. I can tell you a few things about it, Shikoku and Tohoku have been the bane of my existence recently.
For Japanese regions, I usually scout on YT to see if I stumble across something I like, look up the artist, and pray for them to not be from Kanto or Kansai. At some point, I also looked for [insert region/district/city] artist on last.fm. It worked well in the sense I got a list but most were old or very popular stuff I already knew about.
On Wikipedia I also search for
https://en.wikipedia.org/wiki/Category:Singers_from_[insert city]
For France though, you probably already did, but I'd suggest googling "artiste brestois/rennais/quimpérois/whatever else", and maybe add the genre you're looking for. There's good chance you'll stumble across local articles talking about their so-called musician geniuses.
Also, try to look up the guest list of local festivals. I reckon Brittany has quite a few. You have the advantage of the language, so you'll be able to dig deeper.
I know it's not as easy as it seems though. Those who want to be successful in music usually go to Paris, and their origin is not always listed in articles or Wikipedia.