I Am the Uncle from a Japanese Food Shop in Tokyo

I don't understand one thing. I just finished reading the first chapter. The male protagonist opened a shop rashly without any reason, and his taste is normal. When the customers think it tastes bad, they can clearly feel it tastes bad after they leave. How can he have the confidence to open a shop? In this case, why don't you go to a restaurant first and learn some skills before opening it? Generally, this kind of situation is not called normal, it is called psychosis. It tastes as bad as shit. What else is there to blame?

Why is it written in Japan, and then all the characters appearing are basically Chinese? When did Chinese people become so common in Japan?