As of now, if we were to set up interface language, we can not take advantage of the LanguageConverter, which works better in many ways where we don't have to create different page for variations of the same language. It is not possible now to ask LanguageConverter to process interface message based on a default and interface-language. I hope something like a magicword which could process interface message automatically using LanguageConverter.
It's extremely difficult now to have interface message translated, while a lot easier for content. Suppose a template is transcluded in a system message, there does not exist a way where we can localize these templates without duplicating them, which is possible for content.