So far I couldn't figure out how to translate the error messages (perhaps its just the wrong array key), but may be this will help you:
PHP Code:
$this->translate = new Zend_Translate('array', APP_PATH . 'de.php', $this->language);
Zend_Registry::set('Zend_Translate', $this->translate);
Zend_Validate_Abstract::setDefaultTranslator($this->translate);
Zend_Form::setDefaultTranslator($this->translate);