Spain, Portugal and Italy were as well invaded by barbarical hordes during Dark Ages of Obscurantism and Ignorance that followed the fall of the Roman empire.

But we did not renamed our lands after them. Spain remained Spain, Italy remained as Italy, etc.

But the Gauls and Britons totally changed their lands name after their German rulers. What used to be Britain was renamed as England, what used to be Gaul was renamed as France.

I can understand it for regions such as Bavaria, Switzerland, etc., where Latin pre-German citizens were replaced by German villagers. But the population of the previous nations still are majority Celtic.