In BTS they added in some tags in the civinfos that allowed ethnic art styles. Here is a rough tutorial that I threw together. (That one was a bit complex and I couldnt find the better one)
1.Find the file CivilizationInfos in the Assets/XML/Civilization folder.
2. Open it and copy+paste an civilization entry. If you were to copy america, replace AMERICA with SERBIA for all the TXT_KEY entries.
3.You can edit the city list later by copy+pasting the list from the BTS serbia mod.
4.<Scroll Down> You can add different techs and free units by editing the XML tags here.
NOTE: To give serbia a UU, when you get to the UU section, rename it to what you want to UU to be (UNIT_........) and replace the UNITCLASS with what unitclass the unit will belong to (Rifleman, grenadier, knight, ect.)
5.<Scroll Down> When you get to the Leaders, rename them to the serbian leaders.
6. Save it.
7. Open up the file Leaderheads infos.
8. Copy any leader (thats not minorciv or barbarian) and replace all of the TXT_KEY_*leader* with TXT_KEY_*serbia leader*. Repeat process for all leaders you had. Art assets are pretty simple, you can figure it out. Use the leader art from the serbia mod. If you want I'll tell you how to do custom music.
9. I cant remember how to do diploinfos, but if you want I'll check up on it.
10. Open up one of the text files, think it is Objectinfos, and copy+paste tags there and edit the <Tag>(edit here)</Tag> part to match the TXT_KEY tags that you had with the leaders and civ and units. Pedia is in the pediainfos file.
11. Go to unitinfos and copy the unit that has the same unit class as the serbian UU. replace the unit name (not class) with the name you chose back in civinfos and edit the various tags. You can hit Ctrl+f to find a phrase if you need to copy something.
You should be done! Art assets still need to be modified and such.
NOTE: Don't EVER modify the game files, copy the needed files (following the same folder ladder) into the mods section inside the folder 'MyMod' or something like that.
Also, you can use notepad for that. Do a google search for XML marker and if you find a free copy dl it. It tells you when you have an error (forgetting to close a tag, ect) and makes it alot easier.