A working language (also procedural language) is a language that is given a unique legal status in a supra-national company, society, state or other body or organization as its primary mean of communication. It is primarily the language of the daily correspondence and conversation, since the organization usually has members with various differing language backgrounds.
Most international organizations have working languages for their bodies. For a given organization, a working language may or may not also be an official language.
Examples
The United Nations has six official and working languages (Arabic, Chinese, English, French, Russian, and Spanish). German also enjoys special status as a "documentary language". The working languages of the Secretariat of the U.N. are English and French.
Mercosur has two working languages: Portuguese and Spanish.
NATO has two working languages: English and French.
FIFA has four working languages: English, French, German, and Spanish. Formerly, French was the sole official language of the organization. Currently, English is the official language for minutes, correspondence, and announcements.