1. For marinade, crush coriander and cumin seeds in a mortar and pestle. Add ground spices to a large mixing bowl. Chop 4 cloves of garlic and add to bowl with spices. Add juice of 3 limes, 3 oranges, and the pineapple juice to the mixing bowl. Add in white vinegar and 1 tablespoon salt and mix well. Place steak in a glass baking dish and pour marinade over top. Allow to marinate for 8-24 hours in the refrigerator.
2. For potato salad, cook potatoes in a large stock pot of salted boiling water until tender. Once they can easily be pierced with a fork, drain water, and allow potatoes to cool. Preheat oven to 425°F. Lightly oil a rimmed baking sheet and place whole bell peppers on baking sheet, with stems facing sideways. Roast in oven for 20 minutes, flip with tongs, and roast for an additional 20 minutes. Allow to cool slightly before dicing, discarding stems. In a large mixing bowl, add remaining chopped garlic. Add sliced red onion and remaining salt. Mix together. Add sugar, garlic powder, granulated onion, smoked paprika, red wine vinegar, mayonnaise, and cooked potatoes, and mix well. Season with salt and black pepper as desired. Chop one bunch of cilantro and mix into potato salad. Chill in refrigerator until ready to serve.
3. To cook steak, remove flank steak from marinade and discard remaining marinade. Season with salt and black pepper before cooking. Preheat grill or cast iron grill pan until hot. Cook steak to desired doneness, or until internal temperature reaches a minimum of 160°F. Allow steak to rest for 10 minutes before slicing.
4. To serve, spread potato salad on a serving platter to serve as a bed for the steak. Top with sliced flank steak and garnish with remaining cilantro and lime wedges.