You can use the tool provided (tools/nx-pak-tool.exe) to open game's data.pak and see what's inside it.
If you want to create a mod of sorts, all you need to do is copy the files with changes from data/ to data-mymod/ and create a pak from that folder and copy it to Crimsonland install folder. (You can also just copy the folder data-mymod to installation folder, no need to .pak it in development phase.)
When you want to run the game with the mod, use command line parameter --mod data-mymod to run the game:
crimsonland.exe --mod data-mymod
crimsonland.exe --language fi --mod data-mymod