The place I grew up in was right next to a border which is a river, during the summer me and my friends would sometimes swim to the other side to get an ice cream and a coke, now over there I could still speak in my mother tongue, they didn't have a different culture really they were pretty much the same as me and no one was mad that a few youths didn't care much that they crossed some imaginary line.

'Nationalities' and 'cultures' are often even younger than literally '19th century political philosophy' like the German empire didn't exist until 1871 so someone like Karl Marks wasn't German he was Prussian at least according to his nationality, France had to spend centuries to eliminate other languages and there's still pockets of speakers of different languages in it speaking Basque, Occitan and even Dutch, this is such an ignorant take that only a yank could come up with because you live in an actual wasteland and it's not even true for your own country you just never bothered to look further than the little kingdom that is your lawn in suburbia.