Localization of a $description

On line 145, the following code :

    $description = variable_get("captcha_description_$lang_code", t('This question is for testing whether you are a human visitor and to prevent automated spam submissions.'));

seemed to prevent drupal from selecting my translated string. I changes the code to

    $description =  t('This question is for testing whether you are a human visitor and to prevent automated spam submissions.');

and now the translation appears normally.