John McWhorter from The Great Courses: LinguisticsLecture 15: Dialects—Where Do You Draw the Line?
Dialects of one language can be called languages simply because they are spoken in different countries, such as Swedish, Norwegian and Danish. The reverse is also true: The Chinese "dialects" are distinctly different languages.Lecture 16: Dialects—Two Tongues in One Mouth
Diglossia is the sociological division of labor in many societies between two languages, with a "high" one used in formal contexts and a "low" one used in casual ones—as in High German and Swiss German in Switzerland.
Detlef Stern