Combine the cilantro, olive oil, oregano, garlic, lime juice, orange juice, tomatoes, onions, bell peppers, 1 1/2 teaspoons salt and a few grinds of pepper in a food processor and pulse until it forms a chunky sauce. Add it to the slow cooker insert along with the chicken thighs, and toss together. Cover and cook on low for 5 hours.
After 3 hours, add the sweet potatoes, nestling them underneath the chicken. About 30 minutes before the stew is done, add the kale.
Divide the stew among four bowls and serve with hot sauce on the side if desired.