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Simple: open The Grand Campaign.eug file (if you want to play GC), and type the nation you want to play instead FRA or SPA or any other country. You can get names of all countries from:

http://home.hetnet.nl/~copia-fs (Orjane's FAQ)

And save the file, preferably under different name. Launch the game and choose your scenario.

Or just download IGC 1.08, they have a small program that let's you choose your country.
 
Scroll the screen whole way up: you'll see Scenarios button in top of the screen. Click it and select IGC (Improved Grand Campaign). And you got it!
 
I have an English version and works fine for me. So don't worry :)