I should add that much has changed since the document has been published. Mostly for the better, I hope – we now have better documentation at least for some parts mentioned in the document.

From T186591:

I got the page contents in a mixture of Swedish and English. We need to register the tool with translatewiki.net in order to allow the remaining messages to be translated. Since this , by default, only imports the en.json and qqq.json we should then also talk to them to see if the inherited translations can be imported. If there are any messages from video2commons which we don't use any more than these should be stripped out before.

All translations from <languagecode>.json that have matching keys with en.json are imported automatically. And yes do remove unused messages first.