public $validationDomain  = 'validation_errors';

public $validate = array(
    'currency' => array(
        'notempty' => array(
            'rule' => array('notempty'),
            'message' => 'Select a currency'
        ),
    ),
    'title' => array(
        'notempty' => array(
            'rule' => array('notempty'),
            'message' => 'Fill out a title'
        ),
    )

... etc
)

cake.bat i18n extract

./Console/cake i18n extract --validation-domain validation_errors