apt-get/locales errors

[Christoph <csae2692@uibk.ac.at>]

when installing (apt-get) packages or running them
I always get these errors:
dunno how to actually solve it because I am not familar
with the locales topic.
can anyone give me some links/manpages/papers on that?
(not too detailed at the beginning..)

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = (unset),
        LC_ALL = (unset),
        LC_CTYPE = "de_AT@euro ISO-8859-15",
        LANG = "en_EN"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

thank you