this SetFlagTexture "usa_vlajka.pac"
is the code for the U.S. flag that's already in-game
and if you want to add any of the other ones.. you'll need to grab one of the flag packs that Armstrong mentioned.. when you get the custom flag pack, you just need to place a copy of the flag inside your mission folder and then use this bit of code on the flagpole's init field:
this setflagtexture "nameofflag.jpg"
any .jpg will work, it just needs to be the same size as the flags in the flag pack