In Visual Studio 2005 HTML validation schemas directory (Program Files\Microsoft Visual Studio 8\Common7\Packages\Schemas\HTML you can find file i18Languages.xsd that contains list of languages that VS intellisense shows for the lang attribute value. I personally dutifully typed all of them. However, I am curious how many of them actually exist in real Web pages so I decided to begin collecting pages in exotic languages. If you happen to find a page in an interesting language, please post link in the feedback. Thanks a lot :-)
BTW, i-klingon is, apparently, deprecated and changed to 639-2 tlh. So I need to file a bug :-).