{capture assign="var"}{tr}...{/tr}{/capture}
{$var|truncate}

{truncate}{tr}...{/tr}{/truncate}

function do_translation($params, $content, $smarty, &$repeat) {
  if (isset($content)) {
    $options = $params["options"];
    $content = yourTranslateFunction($content);
    if ($options['truncate']) $content = yourTruncateFunction($content);
    return $content;
  }
}
$smarty->registerPlugin("block", "tr", "do_translation");

{tr truncate="1"}Really long text I want to be translated then truncated{/tr}

$smarty->registerPlugin('block', 'tr', 'do_translation', true);
$smarty->registerPlugin('block', 'truncate', 'do_truncation', true);

{truncate}{tr}Really long text I want to be translated then truncated{/tr}{/truncate}