"babel" is a Python module, one of the requirements that Odoo has, as listed in the requirements.txt file.

https://github.com/odoo/odoo/blob/12.0/requirements.txt

https://github.com/odoo/odoo/blob/13.0/requirements.txt

Babel==2.6.0

See the section "Dependencies" at:

https://www.odoo.com/documentation/12.0/setup/install.html#setup-install-source

https://www.odoo.com/documentation/13.0/setup/install.html#setup-install-source

If you don't want to install using the documented method, you can search your favorite search engine for help to install Babel.

I had a similar problem when trying to start odoo from the CLI. It was because I had not activated the virtual environment before starting odoo.

this is the command: source /bin/activate

I'm having the same issue on centos and when I try to install babel it gives me this 

Requirement already satisfied: babel in ./odoo12-venv/lib/python3.6/site-packages (2.3.4)

Requirement already satisfied: pytz>=0a in ./odoo12-venv/lib/python3.6/site-packages (from babel) (2016.7)