The target language should dominate in writing exercises!
... instead I write too many English translations of sentences that I should write in French for example. It's French I'm trying to learn, not English. It is a bit a waste of energy. Well, not completely...
I agree with you. As the sentence structure of the target language increases in complexity, It has become much more challenging translating into that target language. I do think being asked to do more native to target language translations would help reinforce the foreign sentence structure.
I think it would be incumbent on Duolingo to acknowledge the deficiency and attempt to address it. Or create a mechanism for the user to adjust the direction of translation in drills. On the other hand, I do use my own additional software to create drills into the target language with the Duolingo vocabulary. In the long run, it will affect the success of users - which is not nearly as scientifically proven as Duolingo suggests.