BUG/MINOR: stats: add lang attribute to html tag

BUG/MINOR: stats: add lang attribute to html tag

The "html" element of the stats page was missing a "lang" attribute.
This change specifies the "en" value, which corresponds to english
language.

It is also a required element for WCAG Success Criterion 3.1.1, which
renders the web more accessible through a set of requirements. In this
case it allows assistive technologies such as screen readers to
determine the language of the page.

MDN page: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang
HTML standard: https://html.spec.whatwg.org/multipage/dom.html#attr-lang
WCAG criterion: https://www.w3.org/WAI/WCAG22/Understanding/language-of-page.html