By default, ReadCube does not come with Asian characters installed, making you by default unable to type Chinese and Japanese within the client. Fortunately, there's an option within your preferences menu that enables you to install an Asian Language Pack, incorporating these characters into your ReadCube client.
Once installed, you can use Japanese/Chinese/Keoran characters to complete searches in our search engine, look through your library in Asian characters, and enter Chinese/Japanese/Korean comments. You can do this either by using an appropriate keyboard or by using IME, which allows you to enter Asian characters with an English keyboard. You can learn more about IME here.