register_block

/* PHP */
$smarty->register_block("translate", "do_translation");

function do_translation ($params, $content, &$smarty) {
    if ($content) {
        $lang = $params['lang'];
        // do some translation with $content
        echo $translation;
    }
}

{* template *}
{translate lang="br"}
   Hello, world!
{/translate}