When passing parameters values to the XSL engine that are string constants, those values must be quoted twice.
<$names = "lang">
<$values = "'EN'">
<$result = (xsltApplyStylesheet($doc, $style, $names, $values))>
The values get two sets of quotes around them, both double and
single. The Vortex assignment consumes the outer double quotes,
which sets the Vortex variable $values to 'EN'. When
the variable is referenced in XSL using
<xsl:value-of select="$lang"/>, it resolves to
<xsl:value-of select="'EN'"/>. This selects 'the string
constant EN', which the desired behavior.
If the Vortex variable is set as <$values = "EN">, then
Vortex consumes the quotes and the Vortex variable $values
gets the value EN. When this is accessed in XSL using
<xsl:value-of select="$lang"/>, it resolves to
<xsl:value-of select="EN"/>. This selects 'the contents
of the element <EN> in the XML document being transformed'.