I want to use ISO-8859-15 instead of UTF-8 because I use IRC a lot.

For some reason the tip I posted a long time ago does not work in Ubuntu Karmic Koala (the trick was to add the correct locale and then selecting it from GDM).

This seems to do the trick. After adding your locale to /var/lib/locales/supported.d/local like so

en_DK ISO-8859-15

regenerate locates

$ sudo locale-gen --purge

The next step is to change the locale globally

$ sudo update-locale LANG=en_DK

(this will in fact change the LANG variable in the file /etc/default/locale).

Once done, log out and select the newly added locale from GDM’s language selector (there will most likely be two identical entires, select the one not currently selected) – Oh, and you might need to reboot.

Select new language