First you need to install the locales you want:
sudo dpkg-reconfigure locales
Then refresh your current environment:
export LANGUAGE=en_US.UTF-8
export LANG=en_US.UTF-8
export LC_ALL=en_US.UTF-8
sudo locale-gen en_US.UTF-8
If you see these errors (on Jessie Lite) follow the next step
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory
Open your environment file
sudo nano /etc/environment
Add these lines
LC_ALL=en_US.UTF-8
LANG=en_US.UTF-8
Ctrl+X, Y and Enter to Save
That's it.