Complex sentence: Most Shanghai residents are the descendants of immigrants from the two adjacent provinces of Jiangsu and Zhejiang who moved to Shanghai in the late nineteenth and early twentieth centuries, regions whose population speak different dialects of Wu Chinese.
Split into simpler sentences: Most Shanghai residents are the descendants of immigrants from the two adjacent provinces of Jiangsu and Zhejiang who moved to Shanghai in the late 19th and early 20th centuries. Those regions whose population speak different dialects of Wu Chinese.