Changing Back-end Language

Hello and good day.

Can anyone explain to me how to change the back-end language of WPMU? There are plugins available, but I’ve encountered problems with them, like posts made in another language are stored in another locale, but I want them to be unified.
There are how-to tutorials out there as well, but maybe I’m too noobish to understand how to do it.

In any case, can anyone tell me how to change the back-end language from English to Japanese? Or if possible, add an option to switch languages? I only need the Admin Panel language to be changed.

Thanks and good day~