The soft diplomacy of Korea’s first lady, Kim Jung-sook, will be put to the test as she accompanies President Moon Jae-in on his first overseas trip since assuming office last month. Kim, a former member of the Seoul Metropolitan Chorus, has managed to gain a strong fan base here among Moon’s endorsers, who address her as “Cheerful Ms. Jung-sook” due to her hearty laugh and bright demeanor. The anecdote about how she and Moon married is now well known. Moon told a local broadcaster earlier this year that he was hanging out with his friends one day when Kim approached...