how to change a guest's language based on the query string?

I've got a website in English, French and German and I'm adding a forum to the site. The visitor selects their language from a drop down list on any page on the site to switch, and I would like if possible to avoid making them have to manually change the display language for a second time when they visit the forum. It would be nice if I could do something like http://www.abc.com/forum?lang=de to switch the display language to German.

Gary