use strict;
use warnings;

use Template;
my $config = {};
my $tt = Template->new($config);
my $input = "foo.tt";
my $vars = {
    lang => {
        de => {
            "ok" => "Okay",
        },
        en => {
            "ok" => "Ok",
        },
    },
    language => "lang_de",
};
$tt->process($input, $vars)
    or die $tt->error();

__END__
foo.tt:
[% code = "ok" %]
[% mylang = language.substr(5) %]
[% lang.$mylang.$code %]