FRANKFURT—As head of Dutch insurer Aegon, Alexander Wynaendts led a complex European financial institution with staff around the world and a large U.S. presence during a turbulent decade, an experience that should serve him well as the next chair of Germany’s Deutsche Bank. On Friday, a committee of Deutsche Bank’s supervisory board nominated Wynaendts to oversee Germany’s largest lender from next year. The full board backed him at a meeting on Sunday, and shareholders will vote on his appointment in May. If elected, the position will catapult Wynaendts, who is relatively unknown in Germany, into a role as one of the country’s top bankers at a time when Deutsche is also steadying itself after a rocky decade with a view to a possible future merger. Deutsche Bank Chief Executive Officer, in a note to staff on Sunday disclosing the full board’s support for Wynaendts, called the Dutchman a true European …