To prevent keyboard layout breakage for non-English keyboards, xkb-data or xkb-data-i18n is needed for plasma. Rik posed the question if this should be a direct depends or a meta data depends of kubuntu-desktop.
Detailed e-mail of the issue:
Martin Gräßlin <firstname.lastname@example.org>
Jan 22 (6 days ago)
I noticed today that on some distributions the translations of the source package xkeyboard-config  are not installed by default. The translations are essential for the keyboard layout configuration. Without it all layouts are only provided in English which means a non-English speaking user is probably unable to configure the keyboard.
In both screenshots the KCM is loaded for German translations. In the one the xkeyboard-config translations are missing and all layouts are English. In the other case the xkeyboard-config translations are present and all layouts are properly localized (except for the typo in "Englisch").
I kindly request that you check whether your distribution installs the translations and if not to fix this. I assume that this is not only relevant for Plasma, but for all desktop environments providing keyboard layout configuration based on xkeyboard-config.
To give you the information I have. On Debian based distributions the package in question is called:
The relevant translation files are installed to:
On e.g. Ubuntu and derivatives (the distro on which I noticed the problem) the translations are provided by xkb-data-i18n and not installed by default.