OK, I am unsure on what you mean by assigning responsibility to nations.

Generally, when we say Germany is responsible for the Holocaust, this is a bit of a short-hand in the sense that it refers not only to (formerly West-)Germany being the "official" successor state to the Third Reich but also in that with Germany, it was the German leadership of the time, the institutions of the state, as well as broad swaths of society who in one or another initiated or participated in the Holocaust. We assign responsibility to German and nowadays, also to the other successor state of the Third Reich, Austria, because arguably large parts of German and Austrian society had back in the day participated in Nazi policies and this left a cultural and social legacy because it invariably had an impact on these societies. Not because of some concept of "collective guilt" that makes the German and Austrian people (people here in an ethnic sense) responsible for the Nazis.

The cultural and social legacy of this past in Germany and Austria could and can still be felt in the re-integration of Nazi criminals into society, in how institutions of these states dealt with their past, and in how national identity needed to rebuilt upon some form of rejection of the National Socialist past: in East Germany as the "other" Germany of Socialism, in Austria as the first victim of Nazi aggression, and in West-Germany as the new and improved democratic Germany.

With Eastern Europe, this for large parts becomes difficult because most territories in Eastern Europe were under Nazi occupation and therefore it is difficult to state that Eastern European societies at large participated in the Holocaust since as occupations are wont to do, they shatter societies and lead indeed in some territories in Eastern Europe to something akin to a civil war.

Poland for example did not have a government at the time, it had not official political institutions that were able to participate in this policy and while there were collaborators in Poland, there was also one of the largest resistance movements in WWII in form of the Home Army. In Yugoslavia, there was the collaborationist Ustasha regime but at the same also one of the biggest resistance movements in Europe (1943: 300.000 troops, 1945 as many as 800.000 troops in the Yugoslav Partisans).

Even with countries under collaborationist regimes like Bulgaria and Romania, one thing that needs to be pointed out is that the Germans continually pressured these governments into either handing over their Jews to the Germans in order to be deported or pressured them to get rid of them themselves. We assign responsibility to Germany because without the policies by the German leadership and state institutions and without the broad scale participation or at leas acceptance of German society, no large scale mass-murder of the Jews would have occurred.

Now, that does not mean that it is important for virtually every society that once was under German occupation to deal with its history of collaboration and/or the participation of its government in Nazi policies. Some do this, some less so, some still fight about it (just today, a conflict between the Serbia and Croatian government erupted about a movie with the endorsement of the Croatian ministry of culture that claims a lower death toll of Serbs in the Jasenovac concentration camps than is accepted) etc.

I hope this at least kinda answers you question and if you want to know more or had something different in mind, please let me know.