After years of driving a Mercedes-Benz and a BMW, Li Maozai made a choice that surprised even himself: He bought a Chinese luxury car. Mr. Li, a partner at a law firm in the southern Chinese city of Nanchang, chose the Maextro S800, an 18-foot sedan built by Huawei and JAC Motors, for its sleek lines, elegant design and high-tech features that his German cars couldn’t match. At $140,000, it wasn’t cheap, but it was much less expensive than European luxury vehicles. “This car changed our old belief that only…