Linux

For Japanese I use an IME (input method editor), specifically Fcitx. Basically it lets you type in romaji and converts that into kana or kanji. Just note that it needs an additional package (like fcitx-mozc) for each language.

Has anyone successfully typed either European accented characters or Japanese Kanas on their physical keyboard?

For the Latin extended characters, I've used AltGr, Compose, probably at some point the GTK control-shift-u thing. I've also used various emacs text input methods to do so. I don't speak Japanese.

I don't use KDE, but it looks like you can set it up to bind Compose at a per-user level once you've logged into your account.

https://userbase.kde.org/Tutorials/ComposeKey

EDIT: "Motörhead"

that was typed using the Compose key, which on this laptop I have replacing the Right Alt key. On this system, which is Debian, I do it systemwide by editing /etc/default/keyboard, and adding:

XKBOPTIONS="compose:ralt,terminate:ctrl_alt_bksp,ctrl:swapcaps"

That swaps Left Control and Caps Lock, sets Right Alt to be Compose and...hmm, actually, I should check whether Control-Alt-Backspace still functions to kill Wayland, or if that stopped working when I moved off X11.

Then I ran # dpkg-reconfigure keyboard-configuration.

But if you're on a non-Debian-based distro, things may work differently.

for me fcitx works great with KDE, both x11 and wayland. been using it for a long time, with some european languages and chinese

I use the keyboard layout EurKey, which is available on KDE Plasma. It's QWERTY but with European special characters on layer 5, i.e. they are accessed by Alt Gr.