Middle-age Eastern European history can become pretty sad when you think about the huge mixes of peoples in a lot of the areas. Jews, Germans, Armenians, Greeks, and Turks, and Gypsies were scattered throughout, comprising a large percentage of the urban populations. Germans had settled far into Russia, Greeks were the favored civil servants of the Ottomans, Armenians had been wandering since the 14th century but were particularly scattered after the Armenian genocide, and Turks came from the Ottoman Empire, ruler of the Balkans till World War I. Gypsies had been wandering throughout Eurasia since 1000, and Jews have been wandering since biblical times.
Unfortunately, nationalism got the best of cosmopolitan multiculturalism. The story of the Jews is obvious, and the Germans were naturally expelled after World War II, and the ones who were allowed to stay took advantage of West Germany's offer of citizenship to all ethnic Germans and returned. In Romania, first under Gheorghiu-Dej and later under Ceauşescu, the Jews and Germans were actually sold to West Germany and Israel, respectively. Ceauşescu was even reported to have said that "oil, Germans, and Jews" were Romania's best export commodities. In any case, he destroyed most of the oldest parts of Romania's capital, erasing a lot of the evidence that they ever existed. With the coming of nationalism, minorities like the Armenians and Greeks were almost entirely assimilated into whatever the native culture, retaining only strange last names. The Gypsies that were not killed in the Holocaust assimilated somewhat, but their culture has been in some perverse way somewhat preserved due to poverty and racism. However, the use of the language has significantly decreased, and it remains to be seen to what extent Gypsy culture will have survived by the time they achieve socioeconomic parity with the dominant populations. Eastern Europe has become as homogenized as Western Europe.