DATA folder contain all source files from gamedata.vfs and gamelang.vfs (virtual file system) files. 
See the source code (/Core/VirtualFileSystem/ folder) for more info about VFS file structure and how it work.

DATA - should be unpacked

DATA_BASE -/-> gamedata.vfs (partial, do not contain all 3D model/texture files due to license restrictions)

DATA_EN --> gamelang.vfs  // English
DATA_DE --> gamelang.vfs  // German
DATA_RU --> gamelang.vfs  // Russian
