What is the ruling on a person specialized in computer programming who works on a program for buying and selling currencies, knowing that he deals with those who trade with interest-based (riba) companies and with those who do not?

If your work on this program adheres to the necessary Shariah controls for the permissibility of currency trading, then there is no harm in it in principle. Otherwise, it is not permissible, because it is assisting in sin and transgression, as Allah Almighty says: "And do not cooperate in sin and aggression." Furthermore, it is not permissible to pay interest in exchange for the trader holding the currency until the next day (open trade), because it is usury (riba). Therefore, it is not permissible for you to assist these individuals in that.