You probably used: export LANG=de_CH.ISO-8859-1
which sets all LC_* types to de_CH.ISO-8859-1 (execute locale
in a terminal to see this yourself.
To my knowledge LC_MESSAGES is used for command output. You can set this one separately to some other value (POSIX should be fine).
The /etc/profile entry mentioned in chapter 7.8 would then look like this (order is important):
Hope this helps.