Drain pineapple juice into a 1-cup measuring cup. Add oil, lime juice, garlic, red pepper flakes and sugar. Mix well to make the marinade. Place chicken in a large resealable food storage bag. Pour all but 1 tablespoon of marinade over the chicken. Seal bag. Refrigerate 30 to 45 minutes.
To make salsa, combine pineapple, red pepper, red onion, jalapeño pepper, cilantro and reserved 1 tablespoon marinade in a medium-sized serving bowl. Toss well. Season with salt and pepper to taste.
Heat grill to medium. Remove chicken from marinade. Discard marinade. Season chicken with salt and pepper. Grill, turning once, for 10 to 15 minutes or until cooked through and juices run clear. Serve with pineapple salsa.