Thanks to PG1G and PA3BWE, aprs.fi now speaks Nederlands (Dutch)! Thanks guys, that was really quick.
The translation seems quite complete, only a couple of the really long strings are missing. And the Options and Help windows, which are not (yet) available in the translation tool. But this is, in my humble opinion, very cool. Japanese, anyone?
The language shown to the user is selected by the preferred languages list of the web browser (Firefox: Edit -> Preferences -> Advanced -> General -> Languages -> Choose, IE: Tools -> Internet Options -> General -> Languages). The default setting of a browser usually follows the operating system's language settings.