Declaring language in HTML

[xfq]

https://www.w3.org/WAI/WCAG21/Techniques/html/H57.html#resources

It would be useful to add a link to https://www.w3.org/International/questions/qa-html-language-declarations too.

[mbgower]

@xfq feel free to create a PR if you are able to

[bruce-usab]

Mentioned on backlog call as what should be a straightforward PR.

[patrickhlauke]

Filed #4462