I downloaded Quokka Python/Flask CMS to a CentOS7 server. Everything works fine with command
sudo python3 manage.py runserver --host 0.0.0.0 --port 80
If you are trying to execute tests case you must set the following environment variables each time:
export LC_ALL=en_US.utf-8
export LANG=en_US.utf-8
Doing this each time will resolve the error.
It may also be possible to set this in your IDE run configuration as
LC_ALL=en_US.UTF-8;LANG=en_US.UTF-8
For example see the following setting in PyCharm 2016: