Yet somehow even after being cut up, reorganized sectioned etc. The populace was still able to maintain their language, identity and self. I find that fascinating.
"Language, identity and self" are somewhat broad strokes, can you give an example or a definition what you see as Polish/German identity ?
Atleast for the German case, "the" German language and "the" German identity are both relatively modern emergences. The formalization and centralization of the modern German language as a process started with the Grimm brothers around 1800 and large swathes of the "German" identity are tied to the formation of the German Empire by Prussia 1871 (See https://www.reddit.com/r/AskHistorians/comments/34gabd/how_did_prussia_come_to_dominate_all_the_other/ ). So it is a difficult question to answer how much identity has been kept over the last 1000 years.
Short answer? They didn't. Poland and Germany have quite literally been moved over multiple times, with massive population exchanges/forced relocations after WWII. And yes, they do still speak "German" and "Polish", but those languages, like all European languages, have changed radically over the course of hundreds of years. German identity is a rather bizarre case in that, like Cyanocracy said, "Germany" as a nation-state is a very young concept, not existing formally until 1871. Even then, the borders of what constituted Germany were pretty open to interpretation: the first stanza of the Deutschlandlied lists borders covering locations that aren't actually within the modern German state. Poland, however, had a great deal of nation-building done prior to that with the Polish-Lithuanian Commonwealth and their Catholicism letting them find a means of resisting deliberate Orthodox Russification, but -that- is a topic I can't really address: Poland's troubled history is a massive topic and you should find a specialist.
The language and identity of these people was mostly neither Polish nor German. Rather, it was more local and ethnic with the nations serving more as political entities. Poland was largely a coalition of nobles who controlled large tracts of land and who would appoint a king. In that sense it was more a coalition where a strength in numbers was needed, but the identity, and much of the power, tended to be more at the local level.