I think somehow the language to load the page with is being cached. For example, if I set Accept-Language to 'es' and load http://i18n.mbsandbox.org/ I get Spanish. If I change it to 'de' and continue browsing, everything is still in Spanish.

The same problem happens with multiple users on multiple computers - for example, if I load a page requesting German, when reosarevok loads the page requesting Spanish, he will get the interface in German.