There's Frisian dialects still spoken in Northern Germany and in modern Denmark - there are islands dotted from the netherlands Eastwards and northwards jutting up against modern Danish Jutland.
I get the impression that the languages spoken today in that part of the world, did not evolve there. There was a bit of shuffling about of peoples due to the incursion of the Huns which set the Vandals and the Goths a-roving and I believe that set off a chain of events iike a domino chain that resulted in the wandering of the Angles Jutes and Saxons who ended up on the shores of England and France. Basically, the Proto-Norse displaced the West Germanic speaking Jutes, Angles and Saxons Westward.
And I dimly remember reading (or reading dimly) that the Saxons were linguistically assimilated by speakers of other West German languages. I have a copy of the Old Saxon Heliand, and it's basically Old English with wonky spelling.
Do excuse the lack of links, authors, or indeed any real concrete details