I believe the problem here is not having a fallback. I don’t think the translation team will ever be able to be in pair with the coding team and it’s pretty much expected. But to not have a fallback for the keys in, at least, English, make the website unusable.
For the user, a button changing from “Reply” to something bizarre like “pt_BR.Some_Button” will make the user never click on that or even know what that does. Having a fallback to English would be a good change since it’s something the other systems already have and don’t let users hanging for so long. Since it’s in Discourse goal to be the next gen forum system, I believe this should be taken more serious. I’m willing to help.