Let a student from the first team come to the front and write the name of any country in the world, a member of the second team then comes to the front and does the same, the catch is that they can't repeat a country already used.
The game gets progressively more difficult as the number of countries left available runs out. If a team can't think of a new country then they are out of the game, while the others continue until there is only one team remaining, the winners!
You can be lenient on spelling but make sure they don't have access to any maps or geography books which could lead to an unfair advantage.
If the game takes too long or the students find it a little easy then feel free to play the game in a similar fashion to the Animal Alphabet Relay, where the first letter of each country chosen is removed from the alphabet, i.e. if the first team chooses Australia then 'A' is erased from the whiteboard and other countries starting with 'A' such as Austria and Angola can't be used.