Sam Park picks at his lunch at El Torero, a Mexican restaurant in Duluth, a northeastern suburb of Atlanta. It’s a somewhat ironic choice, given that the 32-year-old politician could have picked from dozens of nearby places to emphasize his Korean heritage in a neighborhood where Eastern churches and Asian grocery stores abound. This is home, though. And so the fact that Park knows his tacos as well as his Korean BBQ is kind of the point. Increasingly, the area will be known less as the “Seoul of the South,” than simply the modern iteration of “the South.” Gwinnett County,...