Preheat oven to 425 degrees F. Line a shallow baking pan with foil; set aside. In a small bowl combine cumin, chili powder, salt, and garlic powder. Set aside 1 teaspoon of the spice mixture. Place pork tenderloin in the prepared baking pan. Brush pork with 1 tablespoon of oil. Sprinkle pork with remaining spice mixture, rubbing in with your fingers. Arrange potatoes around pork in the pan. Drizzle potatoes with remaining 1 tablespoon oil and sprinkle with reserved 1 teaspoon spice mixture; toss to coat. Roast for 25 to 30 minutes or until pork is done (145 degrees F) and potatoes are tender. Cover and let stand for 3 minutes.
Meanwhile, in a small bowl stir together yogurt, cucumber, mint, lemon peel, and lemon juice. Season to taste with salt and ground black pepper. Remove pork to a cutting board and slice. Serve pork with potatoes and yogurt mixture.