If you add use diagnostics; it will print a more verbose error message explaining this scenario (there is no variabele @ID which you keyboarded into your string)

string quoting/interpolation are explained in perlintro and Strings in Perl: quoted, interpolated and escaped

You can simplify your xpaths, like this (untested , may contain typos)

 
sub enumerate_languages {
   my( $xmltree ) = @_;
   my @nodes = $xmltree->findnodes( '/language' );
   my @langs = map { $_->tagName } @nodes;
   return @langs;
}
sub get_language {
   my( $xmltree, $lang ) = @_;
   my @nodes = $xmltree->findnodes( "/language/$lang/*" );
   my %pairs ;
   for my $node ( @nodes ){
      $pairs{ $node->getAttribute('ID') } = $node->textContent;
   }
   return %pairs;
}
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FWIW, traditionally this is done using .po/.mo files, each language gets its own file ... gettext.. and all that ... :)