Try one of
LANG=de_DE inkscape LANG=de_DE.UTF-8 inkscape
No, the same error with both.
The available locales are listed in
/etc/locale.gen (Breezy and before)
en_GB.UTF-8 UTF-8 en_US.UTF-8 UTF-8 en_AU.UTF-8 UTF-8 en_BW.UTF-8 UTF-8 en_CA.UTF-8 UTF-8 en_DK.UTF-8 UTF-8 en_HK.UTF-8 UTF-8 en_IE.UTF-8 UTF-8 en_IN UTF-8 en_NZ.UTF-8 UTF-8 en_PH.UTF-8 UTF-8 en_SG.UTF-8 UTF-8 en_ZA.UTF-8 UTF-8 en_ZW.UTF-8 UTF-8
But this makes sense. I don't want a locale, I just want to switch to a different application language, of which I happen to know that it's installed. I fail to see that anything is wrong with the behaviour of my system, and it does what I want.
I certainly don't want to system-wide install the de locale just to have a peek at the German inkscape translation.
ralf