I've been hung up on this for a while, too. I finally figured it out. It's so simple it's not even funny. All you have to do is use php code in the message field and call JTEXT to pass along a language tag.
example:
<?php return JTEXT::_('MY_MESSAGE_TEXT'); ?>
You then have to add your message to an existing language file (preferably one that's already loaded) or create and load your own. You can load a language file in the before form piece like this:
JFactory::getLanguage()->load('my_lang_file');
Next create the appropriate files in the language directories. For this example, I would need files such as these:
/language/en-GB/en-GB.my_lang_file.ini
/language/fr-FR/fr-FR.my_lang_file.ini
/language/es-ES/es-ES.my_lang_file.ini
etc.
Hope that helps!!
Greg