How to change the language used on Mac App Store?
After I fought with the issue for a few hours, I found the solution.


Open Mac App Store and log out of the account.
Restart your Mac.
Log in to the account again. 


You might have to log out of the account first before restarting, for your information.



UPDATE

However, while the solution here worked in my OS X, it's not the case in iOS. In my iPhone, which is set to Italian, the iOS App Store displays English, while my iPad that uses Spanish displays Japanese. This is so frustrating bug in Apple; I don't get to how this can be resolved...