A couple more mozilla-specific remarks on partial translations.
Partial translations is the Fluent feature that decouples Firefox updates from language pack updates. As such, I think this is the selling point that got Fluent into Firefox. All the other reasons are great, but this one is the kicker.
Shipping partial translations is also a cornerstone of our SLA for community translation. There’s sadly only a bit of info on that on mana. Things that need to be a 100% at some point go to vendors now.
Personally, I’d claim: Partial since 2007. Not that I could find slides to back that up 