Enabling Japanese Input Method is always something which takes me more time that it should. Here the short version:
- Install ibus-mozc: apt install ibus-mozc
- Run im-config (it’s interactive)
Log out and back in (all this is only active after re-login). For good measure re-start your X server (Ctrl-Alt-Backspace if enabled).
You should now have the IBus Panel in the lower right corner:
Right-click on it, select Preference, then go to the Input Method tab and add Japanese Mozc. English too if that’s not there already.
Possibly log out and in again. Now you have Japanese Input Method working as expected. Switch between English and Japanese with Meta-Space (usually the left Windows key is the Meta key).
Once you use Japanese, you have to select the correct Japanese input mode (Hiragana in most cases):
Now Konsole as well as Firefox or Chrome can handle Japanese input fine.