Cannot write xml:lang to content.opf

[Xmplar <info@xmplar.biz>]

I have specified an xml:lang value in a book element (i.e. not within an info tag) and need to write that value to the content.opf metadata element.

Here is my customisation:

<xsl:element namespace="http://purl.org/dc/elements/1.1/" name="dcterms:language">
                <xsl:apply-templates
                  select="d:book[@xml:lang]|d:article[@xml:lang]|d:part[@xml:lang]" />
              </xsl:element>

And all I get in content.opf is an empty element:

<dcterms:language xmlns:dcterms="http://purl.org/dc/elements/1.1/" />

What am I missing from the element selection syntax? I'm using 1.76.1, epub2. Thanks

--
Dave Gardiner