Hey Ed,
You could style the messages with css by using :second attributes for the div
But you could also do a template override of tmpl/signed.php and add your classes if you want to have one.
About the language -> that is interesting - it seems that mailchimp is not giving us the default_language info for the list so we fallback to the default value of EN.
I think that the language for the list is set when you go to edit the form in mailchimp and then on the translate it tab. Is it set there?
Regards,
Daniel