English - French language with instructions in French!
Once a person studying French completes the entire tree or reaches a certain level, could you turn all the instructions and explanations in from English to French (or maybe add the option for it)?
It would significantly add to the learning process and is a super easy technical change to make!
Also, please more writing exercises!
As GScottOliver suggests, try the English for French speakers course, which especially at the beginning will give you much more typing in French. It is definitely worth your while, as would be any of the "X for French speakers" courses that you can handle. However, I am fairly sure that you won't hear any French, but only English (or whichever language a course is taught in).
The thing about Spanish for French speakers is that you will end up getting the same instructions over and over. There really aren't very many:
Coche la signification exacte
Écris ceci en français
Écris ceci en espagnol
Écris ce que tu entends
Prononcez cette phrase
Utiliser le banc de mots
Utiliser le clavier
and maybe a few more...
But you will only hear Spanish (as you point out), so if your goal is to hear French it doesn't really aid with that. The main advantage with the reverse tree, I think, is that you are forced to write more sentences in the target language early on.
Given the very little instruction necessary with duolingo--mainly it gives you practice in translating between your language and a target language and you can immediately recognized what is being requested even without reading the instructions--I do not think there would be a major advantage in having the instructions in French.
I agree. Pimsleur introduces instructions in your TL very early. You certainly cannot sit a CEFR exam unless you can read the instructions in your TL!
Writing exercises are vital - but do not work with DL's translate-a-sentence method. The major advantage of DL is the TL sentences are written by native speakers (sometimes the translations are off - but that is another story).