Puree all the ingredients, except the chicken, in a food processor until almost smooth. Piece the chicken with a fork to make tiny holes. Place the chicken in a large shallow baking dish and rub the marinade into the chicken. Cover and refrigerate for 24 to 48 hours, depending on how intense you want the flavor to be. Preheat grill. Grill chicken on each side for 5 to 6 minutes or until cooked through.
For the mango cilantro relish:
Combine the mangoes, onion, cilantro, lime juice, and orange juice in a bowl and gently mix. Season to taste with salt and pepper.