Thanks gordon64, you're right that none of us mentioned kmaps.tcz to be loaded. However, I assumed aug has it loaded already, otherwise
kmap=qwertz/de-latin1
wouldn't have any effect, and I think
All Keys workins in terminal mode [...] but not on the x-desktop
couldn't be true.
I suspect Tiny Core is just a mess to set up for internationalization, as most how-tos available on the net are for the big distros, where one has to just click around in a GUI tool without having to know anything about what just happened to possible config files, additional to LC_* variables.
Personally, I don't want my OS to speak any other language than its default one, except the possibility to understand my commands via my 'non-default' keyboard. Just setting the hole system to another language can lead to confusion (and slowing things down due to loading additional text strings).
For example, in Germany, dates are written as 'Day Month Year' wile in US it's 'Month Day Year'.
Numbers are treated differently, too. Germany uses the dot "." to visualize power-of-thousands, and the comma "," for everything below zero. In US it's vice versa.
So far I wasn't able to configure Tiny Core this way without having issues with my keyboard or certain programs.