Quite a simple one this and only really stops an annoying nag.

Firstly if your american or only want the american English skip straight to step 2, for everyone else

Step 1

apt-get install language-pack-en-base

This pulls down all of the en based language packs

Step 2

I live here in the uk so I want the en_GB but just replace with the en_XX that you want

export LANGUAGE=en_GB.UTF-8

export LANG=en_GB.UTF-8

export LC_ALL=en_GB.UTF-8

(Dont worry if this one reports something like "-bash: warning: setlocale: LC_ALL: cannot change locale (en_GB.UTF-8)" it just means its there already)

locale-gen en_GB.UTF-8

dpkg-reconfigure locales

Step 3

Finally check everything is ok by running

locale

It should report something like

LANG=en_GB.UTF-8
LANGUAGE=en_GB.UTF-8
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
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=en_GB.UTF-8

Problem solved and nagging stops