Language change tends to follow occupation by dominant (invading) culture since it is socially advantageous for natives to conform. In addition, the dominant culture often suppresses local language because they associate its use with rebellion.
Where I live, the Bructeri
were the local Germanic tribe. Then the Saxons came here, then the Franks ...
I don't know what dialect/language the Bructeri spoke or people living here before these arrived. But certainly every of those invaders changed the language.
But place names, for instances, didn't change a lot, not here nor on the British Isles, if they were in common use. Family names stayed as well (my family name is Old Saxon).