When choosing locale settings for Numbers, Time, Currency etc etc, one of the options is Swiss English (en_CH.UTF-8). However when listing locales with locale -a, en_CH isn’t listed, and it breaks how terminal emulators show files or folders with special characters.
The KDE dialogue says “Locale and language files have been generated”, but that doesn’t seem to be true. Is this a bug, or should I manually install en_CH somehow?
Well, it depends on who you are asking 
It is the long standing incompatibility between locale support in KDE/Qt and locale support in glibc/POSIX. KDE supports much wider range of locale combinations than glibc.
See as example
https://bugs.kde.org/show_bug.cgi?id=403580
Which seems to be fixed by since 5.26 by
but openSUSE build explicitly disables this option:
%cmake_kf5 -d build – -DKDE_DEFAULT_HOME=.kde4 -DCMAKE_INSTALL_LOCALEDIR=%{_kf5_localedir} -DGLIBC_LOCALE_GENERATED=TRUE -DGLIBC_LOCALE_GEN=OFF
If no other existing glibc locale satisfies your requirement - yes, you will need to generate and install it. You shuould also open bug report on https://bugzilla.opensuse.org, because now KDE should be able to do it automatically.
So not a bug, just incompatibility between KDE and terminal emulators?
Also, dunno who needs to know this, but im putting it out there. When I changed locale, inkscape(flatpak) was able to open files that previously didn’t work. So if a file.svg was in folder þórður, inkscape was unable to open the file. If I rename folder to thordur, then inkscape was able to open the file.
I will look at how to generate a glibc, thanks!
What everybody might want to know: Leap / Plasma Locales Configuration
For whatever it’s worth, this doesn’t seem to be a problem in Cinnamon with Inkscape.