The first time Donald Trump met Angela Merkel, pictures of their awkward non-handshake in the Oval Office went around the world — and the months that followed did little to break the ice. But if the 63-year-old chancellor sails her way — as widely predicted — to a fourth term in Sunday’s legislative elections, the leaders of the United States and Germany will have to figure out a way to work together in the long term. Pretty much everything about “Mutti,” as Europe’s most influential leader is known at home, sets her apart from the 71-year-old occupant of the White...