Assumptions: you've installed KDE plasma via the profile and kde-plasma/plasma-meta, you run plasma from SDDM, and you're looking to read/write Japanese.
IBus' keyboard layout selection is more limited than KDE's available keyboard layouts.
In addition to rebuilding world packages, you'll need your IBus engine.
IBus has an emoji picker, you'll need the emoji fonts to view it properly. The droid fonts contain glyphs for other languages. Since KDE plasma already has a dependency on noto, these fonts are complimentary.
|Engine||app-i18n/ibus-anthy||See IBus article|
|Non-Latin font||media-fonts/droid||See Fontconfig & Japanese fonts articles|
|Loader||app-i18n/imsettings||setting it up manually|
As an alternative to adding noto-emoji to your world file, set kde-plasma/plasma-desktop[emoji] to pull it in as a dependency.
While you can do it by other methods, I find using imsettings is much easier to set the required environment variables and run the daemon.
test -d ~/.config/imsettings || mkdir -p ~/.config/imsettings
ln -s /etc/X11/xinit/xinput.d/ibus.conf ~/.config/imsettings/xinputrc
Then log out and log back in.