Warning: The html tag has no language attribute
Found by: http://validator.w3.org/i18n-checker/
We need to think about how to handle this.
Our internal default language is english, but (assuming that we have a complete xx translation) if the user requests xx, he'll get all user interface texts in xx.
Another thing is the content, which does not need to be in xx also, but could be just any other language. So I guess we need another lang attr for the content (comes from revision metadata).
Inside the content, there could be yet other languages, so we might need more lang attrs and a way to deal with that.
Transclusions must also be considered.