Ambiguity in treatment of accept attributes

The following sentence in the spec (and the similar ones for the accept 
and accept-charset attributes) is not sufficiently clear:

When the XInclude processor fetches a resource via HTTP, it should 
<http://www.w3.org/TR/2003/WD-xinclude-20031110/#dt-must> include the 
value of the |accept-language| attribute, if one exists, in the HTTP 
request as an |Accept-Language| header as described in section 14.4 of 
[IETF RFC 2616] <http://www.w3.org/TR/2003/WD-xinclude-20031110/#RFC2616>.

Specifically, does this mean the value of the accept-language attribute 
ahould be the *exclusive* value of the Accept-language header? or ca it 
merely be part of it? For example, given this element:

<xi:include href="http://ww.example.com/" accept-language="fr"/>

is it acceptable to send this header:

Accept-language: fr, en *

or must the client send exactly

Accept-language: fr

?
Both versions "include" the value of the accept-language attribute. Is 
this header allowed to include anything else?

A related question: what headers may/must/must not a client send in the 
absence of any or all of the accept attributes?

--
Elliotte Rusty Harold