Non-English Keyboard Input on KDE

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

For the longest time, I've been trying to get non-English characters to appear on my system. Specifically European accented characters. I've read about the compose key, but I could never make it work somehow.

I've also tried to make the Kanas to appear using the Japanese keyboard, but that too doesn't work.

I'm using mostly KDE system, on many different distros. As for the keyboard, it's almost always standard US QWERTY without the numpad, varying between various laptops (mostly Thinkpads) and USB keyboards. For the Japanese, it's a Thinkpad W530 (should also apply to X230, T430, and T530).

I've been using Linux for quite a while now. I'm familiar with most inner working of the system, but this the one thing I can never wrap my head around!