It should be desirable that Zend_Validate_EmailAddress could propagate his current translator to the Zend_Validate_HostName prior validating it.

Also, Zend_Validate_HostName messages aren't returned with the corresponding keys, making them impossible to translate through a Zend_Form object.

I suggest change this code at Zend/Validate/EmailAddress.php from:

// Match hostname part
$hostnameResult = $this->hostnameValidator->isValid($this->_hostname);
if (!$hostnameResult) {
    $this->_error(self::INVALID_HOSTNAME);
    foreach ($this->hostnameValidator->getMessages() as $message) {
         $this->_messages[] = $message;
    }
    foreach ($this->hostnameValidator->getErrors() as $error) {
         $this->_errors[] = $error;
    }
}

To:

// Propagate current translator to the hostname validator
$this->hostnameValidator->setTranslator($this->getTranslator());
// Match hostname part
$hostnameResult = $this->hostnameValidator->isValid($this->_hostname);
if (!$hostnameResult) {
      $this->_error(self::INVALID_HOSTNAME);
       foreach ($this->hostnameValidator->getMessages() as $key=>$message) {
            $this->_messages[$key] = $message;
       }
       foreach ($this->hostnameValidator->getErrors() as $error) {
            $this->_errors[] = $error;
       }
}