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In some ways it's not a bad idea -- with a caveat

Let me point out the good example: Tyrol. For centuries this has been a German-Italian mix. But post WWI it was given to Italy even though South Tyrol was heavily German. The idea to devolve powers to the local government there helped prevent tensions and the locals take control of their own lands

in the case of France and Italy it would be good for Savoy to be the same way and also for Brittany, Gascoigne and Normandy to have the same

The Germans and Spanish and Italians as I noted above have a lot of decentralization

in the rest of Europe each country is a different case -- the smaller ones like Luxemboourg, Liechtenstein or Croatia or Slovakia are already very ethnically specific

Poland had a lot of regional differences and huge minorities but that was ended with WWII and with Poland being pushed physically to the west (losing 1/3rd of it's land in the east) and becoming mono-ethnic.

For the UK, this is already happening with devolution to Scotland, Wales etc. But in England the problem is due to the large numbers of non-English there. I don't know the easy answer to that.