There are differences between Facebook and Discourse. A main one here is that Facebook has what I imagine to be warehouses with banks of servers while most Discourse sites would be running a single server.
I imagine at some point a Discourse hosting provider may have some kind of “enterprise” option that could offer translations without depending on a third party. But I wouldn’t hold your breath waiting for it to happen. Such a feature would be a massive undertaking.
It is one thing to have translations for known UI text (which IMHO Discourse does well), it is entirely different to have translations for random post text content.
I am not part of the CDCK team, but I don’t see this happening as a Core feature anytime soon. And when and if it does, I imagine it would be a “premium” feature to help pay for the extra cost that would be incurred.
IMHO, the best approach for most Discourse sites at this time would be to use the plugin.