In large bowl, combine soy sauce, orange juice, coriander, cumin, turmeric, orange rind, and red-pepper flakes. Add chicken and mix to coat evenly. Set aside 5 minutes to marinate.
In large nonstick skillet, heat vegetable oil over medium-high heat. Add chicken and cook 5 minutes, or until lightly browned. Turn chicken over; add bell pepper and cook 8 to 12 minutes, turning occasionally, or until vegetables are softened and chicken is cooked through. Keep warm.
In small bowl, combine yogurt, cucumber, mint, sugar, and salt. Serve with chicken and vegetables. Serve tortillas warmed, if desired.