When execute, for example, the perl command the message below could be shown:

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

To get rid of this message compile the language locale as shown at “LC_NAME=” text. Compile the nl_NL.UTF-8 locale:

If the message persists check that all locale related environment variables have a valid value (ie they are not (unset)).

Valid values for example:
LANGUAGE="en_US:en"
LC_ALL="en_US.UTF-8"
LANG="en_US.UTF-8"

locale-gen nl_NL.UTF-8