Declaring metadata about the language(s) of the intended audience
In W3C techniques document (Specifying Language in XHTML and HTML Content)
For HTML, should we put language declarations in HTTP headers and meta elements, and how are they different from those in language attributes?
Content-Language in the HTTP1.1 specification (section 14.12)
Content-Language in the HTML specification: only says that the html language attribute has a higher precedence (section 8.1)