Baby Boom of Mixed Children Tests South Korea,” a story by Martin Fackler in the New York Times, reports on the many births that have occurred in recent years to hundreds of thousands of married couples formed by Korean men and women from other countrieses in Asia.    Fackler reports that these are known as multicultural families in Korea, and they tend to be concentrated in the poorer rural farming areas.  Marriages to foreigners were 11 percent of all marriages in South Korea in 2008.