The goal here is to make the English-based system capable of Japanese input, not to make the whole environment based on Japanese.
First, set up locales:
$ sudo dpkg-reconfigure locales
In addition to the English locale (en_US.UTF-8), include Japanese locales (ja_JP.EUC-JP and ja_JP.UTF-8). However, set the default locale for the system to en_US.UTF-8 if you prefer to be mostly in the English environment.
$ sudo aptitude install im-config
Install fonts that include Japanese characters, e.g.,
$ sudo aptitude install fonts-droid
Install and configure IBus-related packages:
$ sudo aptitude install ibus ibus-clutter ibus-mozc $ sudo aptitude install ibus-qt4 # or ibus-gtk3 for Gnome $ im-config
Select “No” to the last question from
Run the IBus setup program:
If asked, start ibus-daemon.
Under the Input Method tab, add -> Japanese -> Mozc.
Under IBus Preferences, configure things to your liking. For example, I prefer making Ctrl + Space (as opposed to Shift + Space) trigger the switch between input modes. The IBus icon also shows up in the system tray, from which various settings can be changed with right click.
Logout the KDE sesssion, restart X session, and login again.