Europa Universalis 3 Wiki

This is where the cultures and culture groups are defined, as an example:

British = {

This defines three cultures; english, american and lowland_scottish, and combines them into a british group. It is worth noting that cultures are not defined based on language, but just as much on other cultural connections. For gameplay they define how easy/difficult it was for a country of one culture to rule over another culture - hence e.g. the placement of Hungary in the West Slavic group, to represent their relatively troublefree rule over e.g. slovaks.

Modding Cultures

You can, of course, change, add or remove cultures or culture groups as you see fit. The names you see in the game are defined in localisation, and you should make the appropriate changes there when changing something here.

Keep in mind that when you use the modded culture in history files, events and so on, you must use exactly the same name as you define here.