Xperia Pro/Neo/V/L :: Japanese Input With Physical Keyboard For It?

I have a Xperia pro (MK16a) So when you're entering text, if you hold down on the screen, you have the option to choose Japanese input and Chinese input. And when you choose Japanese input, you can also open up the setting menu, and inside the menu there are options for physical keyboard, and vitural keyboard. Although Chinese input does not have those settings, but surprisingly I CAN use Chinese input with my physical slide out keybaord. BUT, it is NOT the case with Japanese input. Please give me some guidence on how this works, is it a BUG or something? Because clearly it SUPPOSE to have the option to use my slideout keyboard for Japanese input.