People hear "Slavic" and they think that describes some kind of unified race, with a distinct culture, when really it's just a linguistic classification. The religion, material culture, and social organization of Poland and Bohemia (since Christianity) have been similar to that of the Holy Roman Empire/Germany. The Kingdom of Bohemia was PART OF the HRE, as were many Slavic-speaking lands in what is now Poland. Basically everyone in eastern Germany are descended from speakers of western Slavic languages (Polabians, Sorbians, Wends, Pomeranians, etc) who became acculturated to the German language. There has always been tremendous cultural crossover.
No, they are not identical, but aside from languages, they are far, far more similar to one another than Poland is to Russia. (And even then, Polish and Russian aren't mutually comprehensible). I would say that Medieval Poland and Germany are at least as similar as Medieval Germany and France. We wouldn't say that Medieval France and Germany are exactly identical, but we could very easily group them within the same cultural sphere, to the point that a setting based upon one would be interchangeable with the other in basically any respect except language.
As this Anon mentions:>>52827003
...a setting based upon Serbia or Croatia wouldn't have any real affinity with a "Slavic setting" either. And Medieval Bulgarians would have had more in common with Greeks and Romanians than with any other Slavic people (except for Serbians).
Does pic related look more similar to Catholic European costume and armament, or to Russian? Or look at the old architecture of Poland or Czechoslovakia; it is the same as what you would find in Germany or Austria. (or Hungary, for that matter)