pip install - locale.Error: unsupported locale setting

Question

The root cause is: your environment variable LC_ALL is missing or invalid somehow

Short answer-

just run the following command:

$ export LC_ALL=C

If you keep getting the error in new terminal windows, add it at the bottom of your .bashrc file.

Long answer-

Here is my locale settings:

$ locale
LANG=en_US.UTF-8
LANGUAGE=
LC_CTYPE="C"
LC_NUMERIC="C"
LC_TIME="C"
LC_COLLATE="C"
LC_MONETARY="C"
LC_MESSAGES="C"
LC_PAPER="C"
LC_NAME="C"
LC_ADDRESS="C"
LC_TELEPHONE="C"
LC_MEASUREMENT="C"
LC_IDENTIFICATION="C"
LC_ALL=C

Python2.7

    $ uname -a
    Linux debian 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1+deb8u6 (2015-11-09) x86_64 GNU/Linux
    $ python --version
    Python 2.7.9
    $ pip --version
    pip 8.1.1 from /usr/local/lib/python2.7/dist-packages (python 2.7)
    $ unset LC_ALL
    $ pip install virtualenv
    Traceback (most recent call last):
      File "/usr/local/bin/pip", line 11, in <module>
        sys.exit(main())
      File "/usr/local/lib/python2.7/dist-packages/pip/__init__.py", line 215, in main
        locale.setlocale(locale.LC_ALL, '')
      File "/usr/lib/python2.7/locale.py", line 579, in setlocale
        return _setlocale(category, locale)
    locale.Error: unsupported locale setting
    $ export LC_ALL=C
    $ pip install virtualenv
    Requirement already satisfied (use --upgrade to upgrade): virtualenv in /usr/local/lib/python2.7/dist-packages

Run the following command (it will work):

export LC_ALL="en_US.UTF-8"
export LC_CTYPE="en_US.UTF-8"
sudo dpkg-reconfigure locales

Someone may find it useful. You could put those locale settings in .bashrc file, which usually located in the home directory.
Just add this command in .bashrc:
export LC_ALL=C
then type source .bashrc
Now you don't need to call this command manually every time, when you connecting via ssh for example.

While you can set the locale exporting an env variable, you will have to do that every time you start a session. Setting a locale this way will solve the problem permanently:

sudo apt-get install locales
sudo locale-gen en_US.UTF-8
sudo echo "LANG=en_US.UTF-8" > /etc/default/locale