Thanks @mpy for solving the LANG problem. The answer is to use:
export LANG=en_US.UTF-8
in your .zshrc
.
The remaining problem is caused by the completion system. Unfortunately completion is a monster feature. It involves shell functions or perhaps even scripts being called and somewhere in that process possibly LANG is again set to a wrong value. If you have root privileges you can debug this shell script code). Good luck with the completion guide.