At least as far as I see, there is no feature to switch the language for systemsettings. I do not mean the language that is selected as default language throughout KDE, but the language specifically for the systemsettings application itself. Please add this feature. Reproducible: Always
This might be a problem in XMLGUI framework, omitting the "Switch Language" menu item when adding custom entries to the Help menu. See also bug 355037.
(In reply to Christoph Feck from comment #1) > This might be a problem in XMLGUI framework, omitting the "Switch Language" > menu item when adding custom entries to the Help menu. See also bug 355037. Now it is back there. Sadly I have not the slightest idea what the reason was, because I made very many changes to my system. I do not think it is worth investigating this further unless it should ever happen again. So closing now. Thank you for the hint towards the other bug report. kpat (like many other programs) is here also still missing the entry.