As you pointed out building to an established design would require the approval of the designing country. Germany, Sweden, Italy, France, Russia, South Korea, Japan, and China are the primary builders and I don't see any of them allowing the U.S. to build to one of their designs for Taiwan. Spain is building to a French design so I don't see them as a supplier. So that means starting from scratch. Electric Boat and Newport News are the only builders of subs in the U.S. Neither has built a diesel sub for decades, and both are involved in construction of U.S. nuclear subs so I don't think either one has excess slipways or production capacity. Ingalls and Bath are the builders of surface ships, and neither one has sub experience. Building diesel subs here, to a U.S. design, would be time consuming and expensive. It would require massive subsidies on the part of the U.S. to entice a company like Electric Boat to invest in the infrastructure needed to build a limited number of diesel subs, and which wouldn't be of much use after they were done. With what the subs would cost you might as well gold-plate them while you're at it.
The Spanish S-80 is not a French design-it is based on the joint Spanish-French ‘Scorpene’ sub.Both nations has shared the construction of boats for Malaysia & Chile,though the French are the ones with a majority stake.The S-80’s internals are almost completely different from that of the Scorpene with high American content.Sure,the Spanish may have their issues as well,but that’s one boat which the US can think of yanking away.