Environment variables "LANG*" and "LC_*"

Greetings,

Since  having packages  "sys-apps/locale-gen-3.10"  and "sys-libs/glibc-
2.42-r5"  installed I notice  the following  warning in file  "/var/log/
portage/elog/summary.log":

   WARN: postinst
   locale-gen(8) issued the following warnings:
   perl: warning: Setting locale failed.
   perl: warning: Please check that your locale settings:
           LANGUAGE = (unset),
           LC_ALL = (unset),
           LC_CTYPE = (unset),
           LC_NUMERIC = (unset),
           LC_COLLATE = "C",
           LC_TIME = (unset),
           LC_MESSAGES = "C",
           LC_MONETARY = (unset),
           LC_ADDRESS = "en_US.utf8",
           LC_IDENTIFICATION = "en_US.utf8",
           LC_MEASUREMENT = "en_US.utf8",
           LC_PAPER = (unset),
           LC_TELEPHONE = "en_US.utf8",
           LC_NAME = "en_US.utf8",
           LANG = "en_GB.UTF-8"
       are supported and installed on your system.
   perl: warning: Falling back to a fallback locale ("en_GB.UTF-8").

Up to now my understanding was  that just specifying  "LANG"  would also
set the other unset "LC_*" variables (except "LC_ALL"),  and indeed this
is what "locale" says:

   $ locale
   LANG=en_GB.UTF-8
   LC_CTYPE="en_GB.UTF-8"
   LC_NUMERIC="en_GB.UTF-8"
   LC_TIME="en_GB.UTF-8"
   LC_COLLATE=C
   LC_MONETARY="en_GB.UTF-8"
   LC_MESSAGES="en_GB.UTF-8"
   LC_PAPER="en_GB.UTF-8"
   LC_NAME="en_GB.UTF-8"
   LC_ADDRESS="en_GB.UTF-8"
   LC_TELEPHONE="en_GB.UTF-8"
   LC_MEASUREMENT="en_GB.UTF-8"
   LC_IDENTIFICATION="en_GB.UTF-8"
   LC_ALL=
   $

So what does the message quoted above really want to tell me?

Slightly puzzled
     Rainer