lang() and protocol information

Hi,

  XPath 1.0 says for lang(...) that it returns false if there is no
xml:lang attribute in the document. XML 1.0 Second Edition has an errata
item (E49) to consider protocol information for language determination
(the Content-Language header in HTTP and MIME for example). Does this
affect the lang() function in XPath? Could the errata clarify this
issue?

regards.