Changing OS X command line language
Terminal and iTerm set LANG based on the region selected in System Preferences (defaults read -g AppleLocale) by default. It affects error messages and (some) help messages shown by bash, but not the language used by nano or for man pages.

$ export LANG=de_DE.UTF-8
$ ashuhuf
-bash: ashuhuf: Kommando nicht gefunden.
$ help unalias
unalias: unalias [-a] Name [Name ...]
    Entferne jeden Namen von der Aliasliste.

    Optionen:
      -a    Enferne alle Alias Definitionen.

    Gibt immer Erfolg zurück, wenn der Name existiert.


Adding something like export LANG=de_DE.UTF-8 to a configuration file like .bash_profile would have the same effect. I don't know any way to make the shell environment more localized though.

If others were searching for how to make Terminal or iTerm use English, add for example export LANG=en_US.UTF-8 to .bash_profile. Disabling setting locale variables automatically would make shells use the default C locale, which makes working with non-ASCII characters difficult at least in bash and other programs that use readline.