In ETW there are several cultures in the DB tables, such as european, which are divided into subcultures, such as sc_european_east. Every faction belongs to a culture and subculture, both of which effect the how various things are displayed on the strat and battle maps.
The culture controls which portraits, agents, bodyguards, buildings effected by government type, and strat map buildings each faction uses. It also controls which traits a faction belonging to this culture can't get.
The subculture: controls which sounds are used; battle map cities, battle map bridges, and farms are used; and which ancillaries factions belonging to this subculture can get.
Part 1 Adding a new culture
Part 2 Adding a new subculture
If you just want a new subculture you can skip the first part and add your new subculture to an existing culture.
Part 3 Updating the sound files
Part 4 Adding portraits
You can skip this part if you're just adding a new subculture.
Part 5 Adding building names
You can skip this part if you're just adding a new subculture.