How to Permanently Change Locale to Default in Ubuntu

To find the current locale of the system, simply type in the terminal
$ locale

This will display the current system's locale variables. In order to change the locales to the default values and set the language as US English, you will need to edit the /etc/default/locale file by
$ sudo vim /etc/default/locale

Edit the file with the following text:
LANG="en_US.UTF-8"
LC_ALL="C"

You can reboot to see it works!
For further info, please refer to https://help.ubuntu.com/community/Locale