The <html> element on wikipedia.org is set to lang="mul" and doesn't have a dir attribute. In a certain technical way this is right because there are multiple languages there, but it's not actually correct and must not be done.
The language of most of the page is specifically known and it is the same—that's the language that shows the names and the descriptions of Wikimedia projects, the language of the words "The Free Encyclopedia" at the top, etc. This language's code should be set in the lang attribute of the <html> element and the dir attribute must be set to that language's direction.
The names of the languages of the large and the suggested languages around the globe must be in elements with their own lang and dir elements. If the legal text at the bottom is intentionally left in English for legal reasons, this is reasonable, but then it must be explicitly set to lang="en" dir="ltr".
Not doing this causes real problems. For example the footer-description is shown in Hebrew with incorrect punctuation and direction.