Well, as there are too many countries in the world to list them all, perhaps it's better to list only big regions and countries or continents: Europe, Asia, USA, Canada, Australia and New Zealand, Africa, Mexico, South America, Russia, Turkey, Middle East, and of course "Others", in case I forgot something...
Of course, the European countries are very different from each other, you could divide them in for example Northern Europe, Eastern Europe, Spain, France, Great Britain/Ireland, Germany/Austria/Switzerland, Italy and Greece, but I think that's probably true for every of the mentioned "bigger regions" and thus it gets a bit confusing, so I don't know if that would make sense.
