India’s wildly popular digital payments system, which began as a way for people to send money instantly by phone, could help the country turn one of its biggest domestic technology successes into a tool of diplomacy, analysts say. That opportunity is coming into focus in Indonesia, Southeast Asia’s largest economy, where officials are studying whether India’s low-cost digital systems can be adapted to their own needs. The talks centre on the Unified Payments Interface (UPI), India’s instant...