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I would like to add a new culture/culture group to the game from the main game. Which files should I have to copy and where to do that?
This is really depends on how much work you want to do, while you could just copy the file from Crusader Kings II\common\cultures and cut it down to the group you want you need to keep in mind that 90% of the content in the mod isn't going to work as events, decisions, laws etc are going to be coded to fire only for groups and cultures in the mod. So unless you are willing to review every file in the mod that looks at the culture and edit it to include the culture you are adding if appropriate then I wouldn't bother trying.
 
And to edit the name of a culture? Would that be too difficult?
Because house Manderly being "Former Southron" makes sense from a game mechanics standpoint, but since characters became capable of writing books, it became weird to have one described as "a compilation of Former Southron architectural information" and the like... it reads weird to me. I'm thinking about changing the name for "White Harbor's"/ "White Knife" or something.