Most people around the world speak more than one language.
That shouldn’t be news, but in the English-speaking monolingual world, we may need to remind ourselves of this fact.
One language may be used at home and informally, but in a multilingual world, it’s useful to be able to communicate with people who speak different languages. People with different home languages might share a common language or a ‘trade language’ (especially for the marketplace). These are known as ‘languages of wider communication’. English is obviously one, and so is Hausa (used in northern Nigeria), Mandarin Chinese (for China), Spanish etc. Unfortunately while I can greet people and buy my tomatoes using Hausa, when I try to go much deeper in the language I come up against a problem. Any Language of Wider Communication is also frequently a Language of Wilder Confusion.