It doesn’t matter if Oskarina Fuentes is at a medical appointment an hour from her home in the mountain town of Caldas in Colombia. Nor does it matter if it’s 2 o’ clock in the morning. When her phone vibrates with a WhatsApp alert from her “Task Hunters” group, she has little time to react. Fuentes, 35, rushes to her computer and logs on to Appen, an artificial-intelligence data platform where she has been tagging data for the past decade. She works quickly as she competes with thousands of oth...