Hi,
The first culture group comes down to this : The population was Saxon, mainly, and the rulers of that population where both Frankish and Gothic, (Gaul and Germanic). The Franks (Gauls... I dont know if Frank is an english word, but in the dutch langauge we call them the Franks, and Charles the Great was their king around 800..) were a christian community, that have influenced, by sending missionaries, all the saxon territories to the north. William the Conquerer (1066) also had the Frankish culture. Famous names will be Willibrord who christianized most of the low countries, and Bonifatius who worked in west Germany. The question is : does the culture of a region display the Ruler's culture, in which case western europe will be Frankish/Germanic(Gaul/Gothic). Or does the culture reflect the populations' culture, in which case it would have to be Saxon.
Do not underestimate the Saxons here, their rituals and traditions have been practiced for a long time, and eventually 'merged' with Christian culture, forming the new culture, and eventually resulting in the Protestants... (refusing to accept the Roman church, viewed upon as some kind of "elite" class of foreigners...)
Also, a large part of eastern Germany would also fall under Saxon, should the population determine the culture.
The area around the Mediterranean consists mainly of Roman/Latin influence. These places were the last to fall in the Roman Empire, but their influence remained very very strong there. The knights that fought the morrish people in Iberia were strongly connected to the roman church, and much less to the frankish empire to the north. They only started to develop their own (iberian) culture after they were able to fight back the morrish people and when Leon was founded. So in this period of time, the main culture in Italy and Spain would be Latin.
The "byzantine" culture doesnt exist. This is the Eastern Roman Empire... and basically, this is where the Romans' influence initially stayed the strongest, even stronger than in Italy. Only, their culture slowly merged with Greek and the Saracens in the east. This is the part of the Roman Empire that was not destroyed, and then started a slow decay. That is the reason their culture was influenced strongly by other cultures. A much better description would be "greek" indeed. It IS greek, it's just not the same greek as ancient greek, but greek nonetheless.
Slavic is a good description for most countries in the East, about the same as Saxon would be to describe the population's culture in western europe. Although largely "barbaric" (no offence, but I need SOMETHING to describe it... I know it's not adequate but ow well, Pagan then !

) the German's started to cultivate them (in the north) and the Byzantine empire started to go north, into russia etc...
So I guess, it's whether you want to use population influence, or their ruler's influence. It makes all the difference =)
Greetz
Leonatos