WASHINGTON, April 12 (Reuters) – U.S. Treasury Secretary Janet Yellen on Wednesday called for the World Bank to implement further reforms this year to expand its ability to help developing countries meet global challenges such as climate change. Yellen hosted talks with global finance officials to discuss an initial spate of balance sheet changes that will allow the World Bank to lend an additional $50 billion over 10 years while maintaining its top-tier AAA credit rating, and how to deepen those efforts with it and other multilateral development banks. Yellen said the changes already approved had sharpened the mission of the World Bank to ensure it was striving to end…