1945?
A Völkerwanderung is not possible once structures of government are too stable. You can't have groups of people just moving in and setting up shop once you have that level of control. But until then, such movements happened, which is e.g. why Berlin is now a German-speaking city (not Wendish) while Volgograd is majority-Russian (not Bulgar). If you actually get the authority of the state behind a shift of ethno-geographic boundaries, it would technically still be doable today, though the last time it was done on a grand scale kinda soured everybody on the idea.