ROME — As El Salvador prepares to celebrate the beatification of Archbishop Oscar Romero, the criminal gangs that terrorize the country have agreed to a sort of truce: As a gift to the martyr, they’ve promised to suspend killing police and military officers, judges, politicians, and the poor. Romero, who was shot to death while saying Mass in 1980, is considered a national hero in El Salvador for his defense of the poor and of human rights at the outset of a bloody civil war. His May 23 beatification is expected to be one of the largest public events in...