APA (7th ed.) Citation

Sack, F. L. (1951). The problem of an international language. World Organization of the Teaching Profession.

Chicago Style (17th ed.) Citation

Sack, Friedrich Leopold. The Problem of an International Language. Washington, D.C: World Organization of the Teaching Profession, 1951.

MLA (9th ed.) Citation

Sack, Friedrich Leopold. The Problem of an International Language. World Organization of the Teaching Profession, 1951.