The easiest (and recommended) way to enable Japanese input in KDE is through System Settings -> Regional & Accessibility -> Country/Region & Language. Installing support for Japanese also installs the
scim-anthy input method. If you need to use Japanese in terminal, the easiest way is to set Japanese as system language (but that indeed changes the default language, e.g. most man pages and system commands like apt-get will be in Japanese).
However, this will not enable Japanese input in non-KDE programs like Firefox. For this you need to install
im-switch. Then run
im-switch -s scim-bridge
scim-bridge as default input method for all applications.
scim-bridge is a new socket based input method module that fixes the many annoying problems that were present with
scim, like various crashes in Firefox and Thunderbird and whitespace mapping bugs in KDE. The only thing that doesn’t work is Japanese input in Skype, which is a pity. Apparently this is fixed in KDE 4.0.
All in all this is very painless procedure compared to what it used to be in Dapper or Edgy.