I´ve pulled my hair for several hours when trying to localize the blog engine. So without further due, here´s the solution on Debian-based systems:

First, symlink /etc/locale.gen to /usr/share/i18n/SUPPORTED:
emma:/home/mattias# ln -s /etc/locale.gen /usr/share/i18n/supported
And then run locale-gen:
emma:/home/mattias# locale-gen

The problem is that Debian doesn´t generate all supported locales as default and gettext won´t translate anything if the systems locales is missing. So, either use the solution I provided or run:
emma:/home/mattias# dpkg-reconfigure locales
When asked which locales you want to generate, select "All locales". This will acutally do exactly the same thing as my solution although somewhat more automatic.