In a blender, combine orange juice, pineapple juice, achiote paste, garlic, cumin, oregano, salt, and pepper. Blend until smooth.

In a bowl, combine the sliced pork with the marinade. Cover and refrigerate for at least 4 hours, preferably overnight.

Preheat the grill to medium-high heat. Thread the marinated pork onto skewers.

Grill the pork for about 10-15 minutes, turning occasionally, until cooked through and slightly charred.

Add the diced pineapple to the grill for the last 5 minutes until caramelized.

Warm the corn tortillas on the grill or in a tortilla warmer.

Remove the pork from the skewers and chop into bite-sized pieces.

To assemble, place the pork and grilled pineapple on each tortilla, and top with cilantro, onion, and a squeeze of lime. Serve with salsa if desired.