Compared to the magically easy Japanese input setup on GNOME, setting up Japanese input in Fedora KDE is fairly confusing.
I tried both fcitx-kkc and ibus-kkc. I found switching between 3 or more input methods in fcitx to be very clunky, so I think ibus is the better choice here.
The following steps allowed me to get ibus Japanese working in Fedora 30 KDE:
sudo dnf install ibus-kkc
This should install all the ibus dependencies as well as kkc Japanese input.
Run ibus-setup. This will start the ibus daemon and open a settings window. Add the Japanese Kana-Kanji input method here.
When ibus-setup started, it provided lines that should be added to ~/.bashrc. These suggestions should be followed (without this, switching input methods did not work for me). So add the following to your .bashrc:
export GTK_IM_MODULE=ibus
export XMODIFIERS=@im=ibus
export QT_IM_MODULE=ibus
Activate the configuration:
source ~/.bashrc
Problem: ibus-daemon will not start automatically under Fedora KDE. This problem is documented in this bug report. I followed the suggestions in the report to make ibus-daemon autostart:
sudo dnf groupinstall input-methods
Now run im-chooser, and select "Use IBus". This should make ibus-daemon autostart. Restart and make sure everything is working...
PRs welcome if I've messed anything up in the above instructions.