Setting the language on Debian derived Linux

$ cat /etc/environment
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games"

*** May or may not have the following line ***

LANG="en_US.UTF-8"

$ cat /etc/default/locale
LANG="en_US.UTF-8"

$ cat /var/lib/locales/supported.d/local
en_US.UTF-8 UTF-8

$ sudo dpkg-reconfigure locales
$ source /etc/environment
$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=

If you have issues try running:

$ sudo locale-gen en_US.UTF-8

Then run:

$ sudo dpkg-reconfigure locales