In much of central and eastern Europe German, in various dialects, was a "common language" up to around 100 years ago, even in parts which have now become separate countries with their own languages such the Czech republic and Slovakia. Yiddish, a language spoken widely by Jews in Europe and elsewhere, is a dialect of German with many Hebrew words incorporated.
Indeed Einstein was born in Germany and was educated in the normal German education system. later he worked in Switzerland, where a dialect of German is spoken as one of its 4 official languages (French, Italian, and Romansh are the others).
The more you look at European history the more you will note that present countries and language uses are not old. There were huge changes during the 19th century and after WW1. Even just 30 years ago many new countries were established or redefined in eastern Europe.
Even the USA did not settle a border dispute with Canada along the Alaskan Panhandle until 1903.