Add language indicator to <html>

I think there should be a lang attribute added to the `<html>` tag.

- It's good SEO
- Language tagging is recommended by the W3C Web Accessibility Guidelines
- `:lang()` CSS selector can be used.

Relevant to:

- https://github.com/google/web-starter-kit/commit/9d0eac7775758fab7020342a312a974df98bf349
- https://github.com/h5bp/html5-boilerplate/issues/1542
- https://github.com/yeoman/generator-mobile/pull/62