Locale warning while executing adduser

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http://askubuntu.com – I just tried to add an user to a group $ adduser someuser sudo perl: warning: Setting locale failed. perl: warning: Please check that your locale settings: LANGUAGE = "en_US:en", LC_ALL = (unset), LC_PAPER = "pl_PL.UTF-8", LC_ADDRESS = "pl_PL.UTF-8", LC_MONETARY = "pl_PL.UTF-8", LC_NUMERIC = "pl_PL.UTF-8", LC_TELEPHONE = "pl_PL.UTF-8", LC_IDENTIFICATION = "pl_PL.UTF-8", LC_MEASUREMENT = "pl_PL.UTF-8", LC_TIME = "pl_PL.UTF-8", LC_NAME = "pl_PL.UTF-8", LANG = "en_US.UTF-8" are supported and installed on your system. perl: warning: Falling back (HowTos)