If you want a constant node set rather than one whose contents are calculated by xsl:
instructions, then you can do a trick with document('')
which gives you access to the XML tree of the stylesheet itself:
<xsl:variable name="languagesLiteral">
<lang>en</lang>
<lang>de</lang>
<lang>fr</lang>
</xsl:variable>
<xsl:variable name="languages"
select="document('')//xsl:variable[@name='languagesLiteral']/*" />
This only works for static values, if you had for example <xsl:variable name="foo"><xsl:for-each ...>
then the node set you get from the document('')
trick would be the xsl:for-each
element, not the result of evaluating it.