refactor: update babel build

[12rambau]

As discussed in #1292 the test and build structure is becoming heavier and heavier. This is a proposal change to deal with the specific case of translations.

• Edition of the i18n page (typo + nox commands + titles)
• split of the translation commands in noxfile:
  • translate is running the .pot and .po update
  • add_language is run with compulsory posargs to add a language folder/.po file
• following suggestion from the nox repository I extended the session function to make some session non-compulsory (i.e. they will not be run in a simple nox command). from now on "doc-live" and "add_language" will only be run with nox -s

This modification is required to "free" the nox command. I would like to make some demo of consistent use in local/actions but this need to be done first.

If you like this implementation I would like to create a github action cron job that refresh .pot/.po files on regular basis.

Let me know if I should move forward.

[drammock]

The i18n docs changes look good in code (I didn't check rendered version). I don't understand why the translated strings are being removed from the .po files though; is this related to the recent transifex integration?

The noxfile.py changes look reasonable. Personally I never used nox to run all sessions (because why would I want to run tests, do static doc build, and then live server doc build all at once?) But if you or other devs are in the habit of running nox without -s session_name then I can see why these changes would be wanted.

What this doesn't address is the fact that our CI configs don't make use of our noxfile, so the simplification of .github/workflows/tests.yml that I was hoping for isn't happening here... I guess though that if you add the CRON job then at least the .PO compilation step gets removed from tests.yml right?

[choldgraf]

This seems like a reasonable step forward to me, so generally +1. I haven't looked in depth at the implementation but i don't understand translations infrastructure super well so may not be helpful. I'll hold off on a closer look for when this is marked as ready for review (i have very limited time for open source right now, sure to having a 7 week old!)

[trallard]

This was superseded by #1606, will close this PR