You're mistaken about borders being some kind of fact of human nature, and you're mistaken about different countries having some kind of fundamental "different cultures".

Nation states are an invention of the last few hundred years. Before that, kingdoms had borders between the lands they controlled militarily, and while they would certainly stop an invading army, they didn't have the administrative capacity or desire to stop small scale migration of individuals.

Historically, the culture and language of people living on the Baltic in what is now Germany would certainly differ greatly from the language and culture of people living on the Mediterranean in what is now France. But there was not some magic dividing line between those 2 points, between "the French" and "the Germans". People living around what is now the border of France and Germany would have had a very similar culture and spoken a very similar language.

In the last few hundred years, with the help of such technologies as the telegraph, the railroad, and the radio, nation-building rulers enforced speaking of the Parisian dialect in what is now France, and the Bavarian dialect in what is now Germany. They also created a lot of propaganda and myths about their "national culture" and invented far-fetched stories about the greatness of their national ancestors. In Germany's case, this went way too far and had disastrous results. The French national myths were much more benign, mostly centering around arrogance about cheese.