basicly, XForms is a standard of forms. You can think of it as a replacement for HTML forms (actually, XForms require an XML document to "work" in).
XML as such is more of a data storage facility, with a lot of flexibility.
when you want your form labels in different human languages, that’s simply a servers task, to fill in the appropriate text somewhere. where it gets the required data (DB, JSON, CSV, .ini, XML, etc.), does not matter.