Have the same issue. Mexican N9 with german interface

apt-get always give me the output
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LANGUAGE = (unset),
LC_ALL = (unset),
LC_TIME = "de_DE.UTF-8",
LC_MONETARY = "de_DE.UTF-8",
LC_ADDRESS = "de_DE.UTF-8",
LC_TELEPHONE = "de_DE.UTF-8",
LC_NAME = "de_DE.UTF-8",
LC_MEASUREMENT = "de_DE.UTF-8",
LC_IDENTIFICATION = "de_DE.UTF-8",
LC_NUMERIC = "de_DE.UTF-8",
LC_PAPER = "de_DE.UTF-8",
LANG = "de"
are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

Any advice?