If you manage to bork your system locales in Debian, here’s a fix that works for me.
Open a terminal window and enter the following in sequence…
sudo apt-get purge locales
sudo aptitude install locales
and finally the infamous:
sudo dpkg-reconfigure locales
This should allow you to set things up correctly once again, or perhap for the first time.
This tip was modified from the original on Serverfault