Assuming you have some control over the testing environment, just make your test server part of a network that has whatever IP address range you want to simulate. In the simplest case, you give an interface of your webserver an extra address - an address normaly situated in the country you want to test, and said address to access the webserver.

Note however that using IP addresses to determine which country a person lives is in quite unreliable. People may use proxies located in a different country (specially when using the internet from their work location). Or just because they are a webcafe in country X doesn't mean they don't want the info from their country of residence.

It's even worse if the website decides to adapt the language based on IP number, which no obvious way to reset it. google is an example of bad behaviour in that aspect.