HONG KONG — The International Monetary Fund on Monday approved the Chinese renminbi as one of the world’s main central bank reserve currencies, a major acknowledgement of the country’s rising financial and economic heft. The I.M.F. decision will help pave the way for broader use of the renminbi in trade and finance, securing China’s standing as a global economic power. But it also introduces new uncertainty into China’s economy and financial system, as the country was forced to relax many currency controls to meet the I.M.F. requirements. The changes could inject volatility into the Chinese economy, since large flows of...