I usually set my system language to English, and add French and Japanese as secondary input languages.
I have found the experience out of the box to be somewhat buggy.
Basically, I went over to Yast > keyboard > input
I selected to add French and Japanese,
Somehow what’s happened is my system language has changed to a strange blend of french and Japanese.
This is annoying, because I’m used to english keywords and would really prefer to use my system in English.
To make matters more awkward, that input selection menu did not prompt me to setup the input language switcher, and the input language switcher is nowhere to be found on my desktop:
Hi and welcome to the Forums! I’m not a regular KDE user, but I think you are looking in the wrong place with Yast.
Please check the “Configure Desktop” (AKA “System Settings”) in the KDE Desktop, then in “Personalization” > “Regional Settings” and I think you’ll be able to configure what you need.
AFAIK Yast “System Language” is for X-Windows / Console setup and might be overridden by the desktop settings (it surely is that way with Gnome).