Translation of validation error messages

User.php

<?php

/**
 * Please be aware that __($messageId) is just as function that returns $messageId - it is used for gettext to pick up 
 * message ids.
 */ 
$this->inputFilter
    ->add(array(
        'name' => 'firstname',
        'validators' => array(
            array(
                'name' => 'NotEmpty',
                'options' => array(
                    'messages' => array(
                        Validator\NotEmpty::IS_EMPTY => __('FORM_USER_FIRSTNAME_EMPTY_ERROR'),
                        Validator\NotEmpty::INVALID  => __('FORM_USER_FIRSTNAME_INVALID_ERROR'),
                    ),
                ),
            ),
        ),
    ))
;