on ZF package is present resources/languages where it is possible to find translation for Zend_Validate component. The messages located on Zend_Captcha_Word:

protected $_messageTemplates = array(
        self::MISSING_VALUE => 'Empty captcha value',
        self::MISSING_ID    => 'Captcha ID field is missing',
        self::BAD_CAPTCHA   => 'Captcha value is wrong',
    );

are absent. Is it possible to insert said messages on translation? For italian are:

 // Zend_Captcha
    "Captcha value is wrong" => "Valore captcha non corretto",
    'Empty captcha value'   => "Valore captcha assente" ,
    'Captcha ID field is missing' => "l'ID del campo captcha non è presente",