You mean lingua franca, which is a common language used by speakers of different languages to communicate with each other. Gullah, for example, is a lingua franca spoken by African slaves in America.
The predominant world lingua franca is English.
The lingua franca of India and Pakistan is Urdu/Hindi.
The first lingua franca--seen from a European perspective--was Latin.
The lingua franca in South America is currently Spanish.
A lingua franca is a common language that is used for communication between people of different languages and cultures. Lingua Franca is not a country, nor it is it located in any specific countries.
The current lingua franca is English, but that may be set to change in the next few years.
The lingua franca for India and Pakistan is Hindi/Urdu--which are functionally the same language.
Mozambique is the country you're looking for.
I would guess French.
The Swahili language was developed in 1728 as a lingua franca between Arabs and African traders.
The Lingua Franca status is a descriptive term, and not a fixed status. Lingua Francas cannot be preserved. They change with the times.