Pangaea wasn't a country; it was a vast continent. There were no people at this time. As people spread around the world, (millions of years AFTER Pangaea broke up) they had to adopt different ways to live depending if they were in a desert, at the sea coast, prairies, forests, mountains, etc. These different ways of life gradually became the traditions of those people and were known as their "culture."
Here's a semi-simple answer. Okay: before all of the different countries, there used to be this one massive country called Pangaea. When this broke up to form different countries, gradually people became different because they were split apart. People had different ways of doing things and therefore different languages and cultures evolved :) ( a bit long and annoying but I hope this helped!)