system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why some Europeans countries like France or Spain are these big united countries while eastern Europe is full of all these small separate countries.
gpt
In the past, language/culture wasn't of great important for an country because most common people had nothing to do with the government so they didn't care what language their leader was speaking. But in the 19-20th century this started to change drastically. More people were able to read and implicated in the political system of their country. At that point it become more important for people to have leaders that speak the same language as them. 

So the difference is that some of these country were able to unify the culture/language/religion before the rise of nationalism and others didn't and that could happen for different reason. 

In France for example, the monarchy was strong from early in their history. It was able to develop a big Domain (lang owned by the King) so it had enough money to keep control over the land for a long time. During that time, the seat of government was Paris and the language of Paris became French and spread as the official language throughout the country. France also had strong geographical defences with the Pyrenees in the South, the Alpes in the east and the Ocean on the West, the only big threat was the Germans in the North-East. So it was easier for the king to keep the country together.

Compare that to the West Slavs. Today we talk about Polish, Czech and Slovak. But we could be speaking about one unified country today spreading from Denmark to Belarus, from the Baltic Sea to the Hungarian plains. They are all culturally similar, at least more Similar than the Bretons, French and Occitan were in the past.

But the Czech were always isolated from the Northern plains with mountains ranges. It protected them from invaded, but it also made near impossible any unification of the Western Slavs. Poland did unified most others west Slavs in the region and it became a superpower in the middles ages. But if France and Bohemia (the czech republic in the past) had good geographical border, Poland never had. With Germans from the West and Russian from the East,   Poland was never able to keep control over their territory and if you can't keep control over your territory, then other culture become dominant over yours.