Unfortunately, Joom!Fish can only translate the text if Joomla itself is running in the target language. When executed from a cron job, Joomla will run in the default language. However, if you use the standard message for each reminder type, it will be translated into the language of the client (since that does not rely on Joom!Fish, nBill can go ahead and pick up the value from the language file itself). So as long as you don't need more than one message for each reminder type, you can just customise the language file (reminders.language.php), and use that default text for the reminder message.