Debian apt locale errors

Ok, every once in a while, I play with Debian or Ubuntu machines. Recently, on Squeeze, I started getting errors like these. Probably something with my locale has changed, but I'm not sure what and when.

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perl: warning: Setting locale failed.   
perl: warning: Please check that your locale settings:
    LANGUAGE = "en_US:",
    LC_ALL = (unset),
    LC_TIME = "nl_NL.utf8",
    LC_MONETARY = "nl_NL.utf8",
    LC_CTYPE = "en_US.UTF-8",
    LC_COLLATE = "en_US.UTF-8",
    LC_MESSAGES = "en_US.UTF-8",
    LC_MEASUREMENT = "nl_NL.utf8",
    LC_NUMERIC = "nl_NL.utf8",
    LANG = "en_US"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
locale: Cannot set LC_ALL to default locale: No such file or directory

These errors pop up every time I use apt. Now there used to be a configuration file somewhere in /etc that allowed me to set locales in Debian, I'm pretty sure there was. But it's gone.

Apparently, one now has to edit a file in /var. Not sure why a configuration file ends up there, but hey, I'm not an OS developer. Anyway, you need to edit /var/lib/locales/supported.d/local and add your own locale, in my case "nl_NL.UTF-8 UTF-8". After that, just run

$ sudo locale-gen

like you used to. Very strange place to put a configuration file...