Eastern and Central European history has less stable borders/states than Western/Northern Europeans and Americans often assume. Nationalities existed under various empires without much state-aspirational nationalism until the 19th century. People practicing various religions, speaking various languages lived under changing lords or free in the wild fields, then fled en masse in times of conflict and lived somewhere else. Forced mass-conversions and massacres changed demographics a number of times over the last thousand years. Bureaucracies remained largely intact through their posession by empires, soviets, then nation(ish)-states.

Modern states in the region are real, but contingent, and shouldn't falsely be projected backwards through history.

That's why I mentioned the idea of "locals" - apparently during census in interwar Poland, some rural regions especially in modern West Ukraine had as much as over 20% declaring their nationality as just "locals".