Reading between the lines, it seems obvious that Mozilla wishes to stop funding localization. Translations aren't even that expensive, especially relative to the cost of producing the docs in the first place. It hurts me to see MDN so starved of resources that they can't pay for even that small piece. MDN has been key to so many people's technical education. High-quality web documentation is an essential resource for…
We shouldn't discount volunteer translations. Won't the new platform support that?
Ideally, you would have a master document in which some parts are shared across all translations (like the layout, the actual terms in the spec, browser compatibility information, etc), while other parts were localizable (like the field descriptions). Then when content was added/changed, it would be clear which parts of the translations needed updating, and you could either flag them as such for volunteers, or use machine translation as a stop-gap, or both.
With the architecture they described, however, your options are either to make the translations completely independent or to make translations completely machine translated, neither of which is a good solution.