Coming from Louisiana, I've found myself sadden by the fact that Cajun French is pretty much going to die with my grandparents' generation. It may just be a cultural dialect of French, but it was born from a history that has created the Cajun culture.
However, I'd have to say that I do believe that the world would be better off with a single global language. Removing language barriers has always been the first path to peace. I understand that many people tie their culture with their language, but I think it would help if everyone spoke the same language.
Of course I'd pick English because that is what I speak. I'm no language expert, but it always seemed that English was more adaptable, too.