The HTTP Content-Language header is set on the server and sent with a file.

It can specify more than one language at a time. This is appropriate for declaring primary languages, but not for declaring text-processing language, which can only be a single language at a time.

This declaration is overriden by any declaration using attributes on the html tag.

If no language is declared on the html tag, some, but not all, mainstream browsers recognise the value declared in the HTTP header for text-processing applications. Even in a browser that recognises this declaration, however, availability of this information for specific applications tends to be somewhat uneven.