phranque - 6:38 am on Oct 23, 2012 (gmt 0)

Yes, but I want to know! It seems to be to be the easiest way of doing it in most cases.

the purpose of the Content-Language header is different from the lang attribute.
the Content-Language header is more about the preferred language of the intended audience (i.e. visitor-centric) while the lang attribute is more about the actual language of the document or enclosing element (i.e. content-centric).

the bolded part below is a perfect example if this distinction where the content is in latin but the intended audience is english speakers.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.12

Content-Language The Content-Language entity-header field describes the natural language(s) of the intended audience for the enclosed entity. Note that this might not be equivalent to all the languages used within the entity-body. ... The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user's own preferred language. Thus, if the body content is intended only for a Danish-literate audience, the appropriate field is Content-Language: da If no Content-Language is specified, the default is that the content is intended for all language audiences. This might mean that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended. Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi," presented simultaneously in the original Maori and English versions, would call for Content-Language: mi, en However, just because multiple languages are present within an entity does not mean that it is intended for multiple linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin," which is clearly intended to be used by an English-literate audience. In this case, the Content-Language would properly only include "en". Content-Language MAY be applied to any media type -- it is not limited to textual documents.