use strict;
use warnings;

use XML::LibXSLT;
use XML::LibXML;

my $LANG = 'en';

my $xslt = XML::LibXSLT->new();
my $source = XML::LibXML->load_xml (location => shift);
my $style_doc = XML::LibXML->load_xml (string => do { local $/; <DATA>});

my $stylesheet = $xslt->parse_stylesheet ($style_doc);
my $results = $stylesheet->transform ($source, lang => "'$LANG'");
print $stylesheet->output_as_bytes ($results);

$LANG = 'ru';
my $results = $stylesheet->transform ($source, lang => "'$LANG'");
print $stylesheet->output_as_bytes ($results);

__DATA__
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="1.0">

  <xsl:output method="xml"/>

  <xsl:param name="lang"/>

  <xsl:template match="@* | node()">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()[not(@xml:lang)] | node()[@xml:lang = $lang]">
      </xsl:apply-templates>
    </xsl:copy>
  </xsl:template>

</xsl:stylesheet>