Solución al error perl: warning: Please check that your locale settings:

Más de una vez hemos recibido este mensaje en nuestro Debian/Ubuntu

/etc/cron.daily/man-db:
/usr/bin/mandb: can't set the locale; make sure $LC_* and $LANG are correct
/etc/cron.daily/pve:
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LANGUAGE = (unset),
LC_ALL = (unset),
LANG = "en_US.UTF-8"
are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

$ sudo locale-gen es_ES.UTF-8
Generating locales...
es_ES.UTF-8... done
Generation complete.

Luego reconfiguramos el sistema para que no nos vuelva a salir el error:

$ dpkg-reconfigure locales
Generating locales...
  en_AG.UTF-8... done
  en_AU.UTF-8... done
  en_BW.UTF-8... done
  en_CA.UTF-8... done
  en_DK.UTF-8... done
  en_GB.UTF-8... done
  en_HK.UTF-8... done
  en_IE.UTF-8... done
  en_IN.UTF-8... done
  en_NG.UTF-8... done
  en_NZ.UTF-8... done
  en_PH.UTF-8... done
  en_SG.UTF-8... done
  en_US.UTF-8... done
  en_ZA.UTF-8... done
  en_ZM.UTF-8... done
  en_ZW.UTF-8... done
  es_ES.UTF-8... up-to-date
Generation complete.