<Adrian puts on his usability hat>

I second that, don't do it via IP address. It only annoys people. As well as people who work in one country, but come from another you also have people who prefer to use a particular language that may not be their native one. People also make these decisions on a site-by-site basis depending on the information the site provides, and the strength of its support for other languages.

In fact, I'd go further and say that you shouldn't automate it at all. At least, don't do it unless the user can override it on the web site.

I've done user testing on a few international sites and every single time some users have wanted to change the default language.

Use ACCEPT_LANGUAGE to set the default, but allow the user to override it, or I guarantee some people will send you nasty feedback and you'll have to do it later :-)

<Adrian goes away and tries to remember where he put his perl hat>