Improve documentation for initializing the i18n extension

[akx]

Refs the discussion at https://stackoverflow.com/a/78970088 to make it clearer what to do to hook up i18n.

[davidism]

Remember to make docs changes to the latest .x branch rather than main, so that they show up in the current docs builds.

[akx]

@davidism Retargeted & rebased on 3.1.x.

[myersjustinc]

Thank you for the edit, @akx, and thank you for the merge, @davidism! I'm the person who wrote the SO answer referenced above after trying to navigate this process without this explanation being present, so I really appreciate your help making things clearer for other users down the road.