I have placed the error messages like below in my observer.php
$msg = Mage::helper('mymodule')->__('You had already added the subscription in previous session.Add the subscription again and proceed to checkout');
Mage::getSingleton('core/session')->addError($msg);
and added the translation in
app/locale/es_ES/Mymodule.csv
But the still the messages are not translated. Can anyone suggest a workaround ?