Say, I make a change to En. Then I update Fr based on En. Say it's a pretty long translation.
Since I'm a wiki-translator, I'm translating content that I care about and that I know a lot about. In fact, I am also a prolific author on that site.
While translating the change, I spot an error in a part of the text that has nothing to do with the translation I am currently doing. Chances are that I will go ahead and fix the error right there while I'm still in the middle of a translation transaction.
Then I finish the translation, and I click on Save.
Problem is, the system now considers that En and Fr are in synch, eventhough my correction (which I did at the same time as translation) is not in the En version.
This type of scenario has happend to me (AD) often. Because I knew about the issue, I was usually able to be disciplined about it and just write down the change on a piece of paper, and delay doing it until I was done translating. But even there, I sometimes forgot that I wasn't supposed to do that. So for sure, "ordinary" users will have trouble splitting their personality into "translator" and "author" roles.
So... I think we need to gently prevent people from doing this as much as possible.
Again here, a solution could come from some sort of translattion verification feature. If the system sees a change that looks like it was not a translation of a change on the En side, it would signal that to the user, and maybe allow the user to label that change as being an original contribution as opposed to a translation.