Thanks for the help, I could solve it now. The error message came from usr/share/locale/de/LC_MESSAGES/libc.mo and deleting it (well, renaming for now) did the trick. It's now "bad file descriptor" which was a lot easier to find in google ;-)
I also tried the other stuff, but I failed with that.
Not sure where I could set LC_ALL within c::b, I could not find an environment variables settings in Environment. Setting it manually before start did translate the c::b menus, but not that error message.
Changing system language did for some reason also not do the trick. I've got an English KDE that way, but that error did stay German. Though I only did change the language - not the locale (I didn't want to change stuff like currencies, etc), so that might have worked.
Not sure about that EnvVar plugin, is that something newer than 8.02? I didn't find it.