Verbatim of a message I sent to the HTML WG a while ago:

Could the HTML WG clarify how the hreflang is to be used and set when the target of a link has more than one base language ? This question is not only theoretical and I have already discussed this issue with Karl Dubost. We have live examples where we need this. Could a modification of this attribute, changed from one language code to a list of white-space or comma (no personal preference here) separated language codes, be considered ?

Answer of the Group:

This is a new feature, not an errata. We will address this in a future version of XHTML.

Well, I disagree, my first email's paragraph deserves an errata : the HTML 4.01 spec should at least say that the content of the hreflang attribute is any of the document's languages when the document uses more than one language...

Speaking of the extension I proposed in the second paragraph, only Tantek said it's a good idea and added he prefers a white-space separated list to able to use [ ~= ] attribute selectors. Ian, could you address that in the WHAT WG ?