Are you making this form to submit to a SharePoint list or some other type data transfer. If not, couldn't you just make 2 multiple selectors and only have the US one show if the country select = "us" and have the French one show if the country select = "french".
As far as submitting it, I was hoping I could make a quick form that if "french" was selected, then the french only multi would display, then on pressing the submit button it would copy that information to a AllCountry selector that has all the options. It apparently doesn't quit work that way.
I attached the quick form I did, probably no use to you. Hopefully someone smarter helps you out :) Good Luck!