A while ago, when I was typing ISO 639 language table for Whidbey intellisense on the lang attribute, I found that the list of languages included a lot of interesting ones, like, for example, Klingon. I did find quite some amount of pages written in Kilngon, but none of them actually specified lang="tlh". One page specifies <language="tlh">, but it does not seem to be correct HTML as it looks like nameless element with language attribute.
Now there is at least one page on Wikipedia that specifies xml:lang="tlh".
Beyond that, I found that Klingon codepage is 1554 and here is an interesting post on how to make Klingon culture for .NET Framework. Mozilla added support for TLH about a year ago. And here is Klingon UI for Google.
Web is so much fun 🙂