You can add messages to be handled by the I18nMessageProcessor using the following methods in I18nFormHandler or GearConfigFormHandler:

addFailureMessage(resourceBundleName, resourceKey, argArray);
addSuccessMessage(resourceBundleName, resourceKey, argArray);

You can also use a message processor directly outside of a form handler class. You can add messages to be later localized using the I18nMessageProcessor.addMessage() method and add messages that are already localized or that don’t need to be localized using I18nMessageProcessor.addLocalizedMessage().