In a medium bowl, prepare the relish by mixing together the cucumber, parsley, chopped onion, bell pepper, and red pepper flakes. Set aside.
In a small bowl, mix the cumin and chili powder with the olive oil. Rub the mixture onto the chicken, and place in a shallow dish. Marinate in the refrigerator at least 1 hour.
Prepare the grill for medium heat.
Lightly oil the grill grate. Grill chicken 8 minutes per side, or until juices run clear. Serve with cucumber relish.