Americans are most likely to encounter it among the (guilty-feeling) children of immigrants or near the borders with Mexico or Quebec — it’s when someone can understand a language but not speak it. It occurs in many polyglot cities as well as along linguistic borders; it can also be observed on islands with multiple native ethnic groups. On one island, off the north coast of Australia, whose 500 people speak a total of nine languages, receptive multilingualism is basically mandatory, grounded in deep social conventions.