Hi Henry,
Though from today I would also prefer the way Brenden mentioned but here is a quick and dirty way to do this which I use to set multiple tags using Javascript.
(function() {
document.getElementsByTagName('html')[0].setAttribute('lang', 'en');
document.getElementsByTagName('html')[0].setAttribute('xml:lang','en');
document.getElementsByTagName('html')[0].setAttribute('xmlns:fb','http://ogp.me/ns/fb#');
})();
The first two statements is answer to your question.
I have kept third in case you want to add library to include open graph tags on your website.
By using this simple pure javascript code you may add as many tags you want.
You can put this code in a HEAD HTML webpart. Since it's purely in Javascript it doesn't require need for any other library to be loaded first.
Thanks,
Chetan