I repeatedly ran into the following warnings while trying to update my Debian box using apt-get.
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LANGUAGE = (unset),
LC_ALL = (unset),
LANG = "en_US.UTF-8"
are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory
From what I understand, the fix for desktop versions of Debian/Ubuntu is to log out and in the login window, choose a language (is there such an option?). This should automagically fix this issue. One servers where you're basically working on the command-line, running dpkg-reconfigure locales fixed this issue for moi. Running this command popped up a configuration window where I could choose different locales. I chose the en_us options, one with UTF-8 and another with ISO-thinggymejiggy support, and setting the UTF-8 option as the default locale.
apt-get now worked sans warnings. I rebooted the server for good measure.
Hope this helps :)