jLanguage 2.2 is an internalization plugin from Jakob Jensen at http://jakobj.dk/blog. It allows a user to markup their posts in various languages. Update - 1/27/2010: It appears that Jakob’s blog is offline. There is a newer plugin called xLanguage available from Wordpress.org.
I wanted to extend the functionality so that when the user clicked on the language link to change to the appropriate post translation it would also set the default language of WordPress so the entire site would be translated. The site owner would only need to download the language files from WordPress and make sure their theme is coded to use WordPress’s localization.