The concern I'm trying to express is that the translations of this content will grow increasingly stale as the original articles are updated, and will be missing entirely for new articles. People who don't speak or read one of the top 12 languages will just be out of luck, unless some generous soul decides to do that substantial amount of work for free.

As far as I understand Mozilla's point is that this is already happening. I agree that stuff like the UI should still be localized manually.