"/language/$language/widget[\@ID='$id']"
[download]

The reason why 1 cannot be mistaken for an element name is that element names cannot start with digits. Use "a" and "b" with the following XML to see what is matched against what:

#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML;
my $tree = 'XML::LibXML'->load_xml(string => <<'EOT');
<?xml version="1.0"?>
<language>
    <english>
        <widget ID="a">input</widget>
        <widget ID="b">output</widget>
    </english>
    <deutsch>
        <widget ID="b">
          eingabe
          <b>b</b>
        </widget>
        <widget ID="b">ausgabe</widget>
    </deutsch>
</language>
EOT

test('english', 'a');
test('deutsch', 'b');

sub test {
    my ($language,$id) = @_;
    my @result = $tree->findnodes("/language/$language/widget[\@ID=$id
+]");
    print $_->textContent . "\n" for @result;
    my @result2 = $tree->findnodes("/language/$language/widget[\@ID='$
+id']");
    print $_->textContent . "\n" for @result2;
}
[download]

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

Thanks choroba, I think I've "got it" now. I have updated my original reply.