Google Webmaster Tools reports over 2000 instances of duplicate titles (about 200 unique titles), and most seem to be cases where the page, or at least its title, hasn't been translated.
Idea 1: Pages which aren't translated at all could issue a redirect from www.tuxpaint.org/path/?lang=ll back to /path/. This is a little tough, because we'd need to add a buffering layer, wrap all of the gettext()'s in a function that would determine whether the actual gettext() call did any work, and then decide whether to display the final output, or cause a redirect. (This could be a little annoying with the en_XX locales.) Keep in mind that most users dont' go from one "?lang=XX"page to another; once they hit one, they get a cookie, and pages automatically translate. Crawlers are the ones hitting every page with "?lang" tacked onto the end.
Idea 2: Calls to page_start() (which generates the beginning of the page, including <head> and its