[FR] Check html lang attribute is valid

[r-leyshon]

Once the approach to implementing html language attribute value has been decided, it would be advisable to check that the language value is a valid language subtag. The approach in accessrmd is to check against the IANA subtag registry and to error if the lang value was not found.

• accessrmd::detect_html_lang() implementation uses the IANA subtag registry