Whisk sweet soy sauce, sake, pineapple juice, mirin, reduced-sodium soy sauce, cilantro, sugar, ginger, garlic, scallions, and chile paste together in a large glass or ceramic bowl. Add skirt steak and toss to coat. Cover the bowl with plastic wrap and marinate in the refrigerator for 3 hours.
Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Remove meat from marinade, and shake off excess. Pour marinade into a saucepan; bring to a boil and cook until sauce is reduced by half, about 10 minutes. Strain sauce into a bowl.
Cook steak on the preheated grill until it begins to firm and is hot and slightly pink in the center, about 4 to 6 minutes per side. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C). Transfer steak to a plate and let rest, about 5 minutes. Serve steak with sauce.