Thanks for the info, Mark!
My priority is translating the HTML and email templates that my list users will interact with. Is it possible to install them in Mailman even while incomplete? As far as I understand, I need to make a directory /etc/mailman/eo and put my files there, and then add a line like this to the end of /usr/lib/ mailman/Mailman/Defaults.py:
add_language('eo', _('Esperanto'), 'utf-8')
Am I on the right track?
I suggest you start by installing bzr and obtaining the Mailman 2.1 branch via 'bzr branch lp:mailman/2.1'.
You will need to create the messages/eo/LC_MESSAGES/ directory and the templates/eo/ directory. messages/eo/LC_MESSAGES/ will contain your mailman.po file with your translated message strings and templates/eo/ will contain your translations of the files in templates/en/.
To create your mailman.po, you start with the messages/mailman.pot template file and either add your translated message strings manually or use some tool if you have access to one.
If you have specific questions, please ask here.