On the Flathead Reservation in northwestern Montana, the sixty remaining fluent speakers of Montana Salish, most of them elderly, speak their language only to each other, changing to English when outsiders or younger tribal members are present. The Aleuts who used to live on Bering Island off the east coast of Russia speak Russian in addition to their native Aleut. The Republic of Singapore, an island nation of just 238 square miles, boasts four official languages. Language contact is everywhere: no nation has a completely monolingual citizenry and many have more than one official...