Alright, discussing this issue in #grml Thorsten 'mira' Glaser brought up the 
following idea:

* do not source /etc/default/locale if something like $ZSH_NO_DEFAULT_LOCALE is 
set
* $ZSH_NO_DEFAULT_LOCALE isn't set by default, but:
* iso-term can set $ZSH_NO_DEFAULT_LOCALE and avoid sourcing /etc/default/
locale via /etc/zsh/zshrc therefore

This should prevent us from yet another change in the locale hell.

Frank, what do you think?

regards,
-mika-