> Should I judge all individuals by the country they come from?
Interesting question, but not one that my post circled about. As someone who grew up at the border between three nations/languages I can assure you that even within a 100 mile radius differing history and culture can change how a average person from that nation behaves. This is curious and one can spend a lot of time pondering about where these slight differences come from – and learn a lot about other cultures and maybe even your own on the way.
What one should – however – not do is extrapolating the character of individuals from their perceived nationality (something I never suggested would be a good idea).
> There are no countries, just beliefs.
Citation needed. I also believe that the idea of the nation is indeed a story we tell ourselves. But firstly stories we tell ourselves are incredibly powerful: They shape actual physical reality and individual actions, and secondly the difference in languages does shape our thinking and our actions and this also has been studied. Nation borders are often also language borders.