When US President Donald Trump and Chinese President Xi Jinping meet again in Shenzhen next month, and then in Miami in December, the world will watch for signs of progress on trade, technology, security and other contentious issues. Such high-level diplomacy is indispensable. But the future of the US-China relationship also depends on something less visible: whether their people, especially the young, can still understand one another. As Washington and Beijing navigate intensifying strategic...