“Now, your alternate history is the Celts in Brittany are DIFFERENT FROM the Celts in Galicia?”
So you’re claiming Brythonic and Continental Celtic languages are the same?
http://upload.wikimedia.org/wikipedia/commons/4/4f/IndoEuropeanTree.svg
I have no doubt that most ancient Europeans shared the same DNA, but that doesn’t mean they all shared the same language.
Remember, linguistic differences are more a matter of geography and history than of some inborn characteristic.