How to set LC_ALL=en_GB.UTF-8 in Raspbian?

Edit your /etc/locale.gen then uncomment the following line:

 en_GB.UTF-8 UTF-8

Run:

locale-gen en_GB.UTF-8 UTF-8
update-locale en_GB.UTF-8 UTF-8
export LANGUAGE=en_GB.UTF-8
export LC_ALL=en_GB.UTF-8

Verify it; locale :

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