It produces "en" for me:

use warnings;
use strict;
use WWW::Wikipedia;
use Encode;

my $wiki = WWW::Wikipedia->new();
my $entry = $wiki->search( 'Rotation' );
my @languages_available = $entry->languages();

foreach (@languages_available) {
    print "$_\n";
}

__END__

en
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Maybe you can use Data::Dumper to see what $entry has for you:

use Data::Dumper;
print Dumper($entry);
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