> Or is there a more elegant way? I will be using XSLT 2.0.

That's about it really. As an alternative to going back up each time 
ancestor::doc you could pass it down as you go along


<xsl:template match="@idref | @id">
    <xsl:param name="lang" tunnel="yes"/>
   <xsl:attribute name="{name()}"
   select="concat($lang, '_', .)"/>
</xsl:template>

and use 
xsl:with-param to set this in the template for doc.

However I have no idea on the relative costs of doing ancestor:: or
passing down a tunnel param. Presumably depends on whether you use
an idiom that causes Michael's optimiser to do something clever or
whether the code actually implements the document traversal..

David

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