If you want to avoid extensions, probably simplest is to go

<xsl:for-each select="report">
  <xsl:sort select="document('')/*/month:*[local-name()=current()/@month]"/>

where your stylesheet looks like


<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                version="1.0"
                xmlns:month="foo:month-namespace"
                >

<month:jan>1</month:jan>
<month:feb>2</month:feb>
...


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list