Change the default language in Open Suse Tumbleweed

I installed OpenSuse Tumbleweed instead of Debian, with which I have a 25 years familiarity, on my new PC. Now, I have problems with UTF-8, e.g., when I cat some_file containing UTF-8 encoded Unicode characters I see some garbage, also input methods don't work properly. To fix the problems, e.g., I can start an XTerm like LANG=C.utf8 xterm and in the new XTerm everything is fine. What I want to do is to set my LANG to C.utf8 globally, but every (EVERY) guide I've found just explains how to use Yast to perform the task, and Yast lists tens of different languages but no C. The configuration file /etc/sysconfig/language is over 100 lines and I find it confusing, and I'm not particularly confident that it's the right place to modify. How must I LANG=C.utf8 permanently, using Yast or without using Yast?