How did western europe maintain cultural unity after the fall of the western roman empire?

Probably because of the united hate of the Roman Empire. The empire kind of created two cultures. The on inside it's borders, and the one outside it's borders. Even the Romans themselves would note that the "barbarians" had a similar culture between themselves. Still there were many rivalries among the groups that destroyed the Western Roman Empire, and surprisingly, they used mixed Roman culture with there's.  Hope this helped, have a great day! :D