common language; lingua franca
Refers to a language shared by speakers of different native languages, used for communication between groups. Can be a natural language, a constructed language, or a specialized jargon.
English is used as an international common language.
Let's use Japanese as the common language for this project.
公用語 is an official language designated by a country or organization, while 共通言語 is any language used for communication between different groups, not necessarily official.
母語 is one's native language, whereas 共通言語 is a shared language used across native language boundaries.
Compound of 共通 (common, shared) and 言語 (language).