exoteric language
Definition - A language that is used for between language-group communication (e.g. English, French, and Mandarin) - i.e. a language that is also a ligua franca.

As opposed to an esoteric language, which is used primarily for within-group communication (e.g. Navaho) and is spoken by few if any outsiders.

1. Esoteric languages tend to become more complex over time, hence become increasingly difficult for outsiders to learn. In contrast, exoteric languages tend to become less complex over time as the complexities are worn away by outsiders trying to learn them.

Etymology -
The term was introduced by the linguist William R. Thurston.

Please comment