Germany isn't going to take it back. It already had the chance, I believe, during the 1990s in exchange for a large sum of money but refused. Germany's eastern boundaries are formally set by treaty. Germany does not want jokes about the "return of German revanchism" and "East Prussia". More seriously, Germany would not want to absorb so many Russian speakers.

A small, Russian-speaking nation under EU's influence (and presumably future EU member) would be a good thing for the EU, to show off to the world (and specifically to Russia, as a positive example).