Thanks - here is where that got ne so far...
I have set the modest goal of changing a string on the list subscription page from the form label "Your name (optional)" to "Your name (and organisation)"
What I did was
I edited /opt/mailman/mm/settings.py and changed the LANGUAGE_CODE from 'en-us' to 'en-gb' (or should I do it at /opt/mailman/mm/venv/lib/python3.7/site-packages/falcon/bench/dj/dj/settings.py or another location?)
Then I switched user using "su mailman" and started the venv environment (using: source /opt/mailman/mm/venv/bin/activate)
I edit the po file at /opt/mailman/mm/venv/lib/python3.7/site-packages/postorius/locale/ and specifically I am testing it on editing en_GB/LC_MESSAGES/django.po - is this the correct location?
I run: mm/venv/lib/python3.7/site-packages/falcon/bench/dj/manage.py compilemessages - is this the correct one to run? After this, nothing changed on the list page yet.
After that I changed directory into "/opt/mailman/mm/static" and run python ./mm/venv/lib/python3.7/site-packages/falcon/bench/dj/manage.py collectstatic - but this produces an error about ROOT_STATIC not being set
...and I didn't want to fiddle too much in case I overwrote the current functioning site ;-)
Is the collectstatic static necessary after running compilemessages? Am I using the right files to run it? Am I editing them in the right location?
Cheers,
Duane