http://greenbytes.de/tech/webdav/draft-reschke-webdav-search-latest.html#rfc.issue.JW24d

EJW asked: Where does xml:lang go in a query?

This is answered by text in 5.6.1 (regardless of how the issue in 5.6.1 is resolved)

 julian.reschke replied " What would be the *purpose* of putting xml:lang into a query?"

The purpose is to allow one to express queries more precisely, e.g. to distinguish between the English word "hoop" (a circular object) and Dutch "hoop" (hope).  Imagine a property that holds keywords for a resource.   See 4.4 in http://www.ietf.org/rfc/rfc2518.txt, and 2.12 in http://www.w3.org/TR/REC-xml