nixre.net
In case of the following messages

	perl: warning: Setting locale failed.
	perl: warning: Please check that your locale settings:
	LANGUAGE = (unset),
	LC_ALL = (unset),
	LANG = "en_US.utf8"
	are supported and installed on your system.
	perl: warning: Falling back to the standard locale ("C").

even if the locales are set :

	$ nix-env -iA nixpkgs.glibcLocales

and export the environment variable LOCALE_ARCHIVE as follows

	export LOCALE_ARCHIVE="$(readlink ~/.nix-profile/lib/locale)/locale-archive"


source: https://github.com/NixOS/nixpkgs/issues/8398#issuecomment-186832814