There should be a word in each page/day that is always translated. That way we can reinforce translation of the word. Example: Every time the word "such" pops up, it is always translated ("tel")
Word of the Day/Week
I feel like I keep seeing the same, easy words I already know over and over mixed with the occasional, unfamiliar word I see once and then never see again. Repetition is important for retention. I think it would be a neat feature to have a Word of the Day or Week where every day/week, there's a new word (preferably less-common) that any time it appears on your screen, it'd be translated (like your vocab words but temporary). I, personally, would like this repetition to learn new words on a regular basis.
Higher translation count
This would allow the user to start out only at a few words per page, and increase until nearly the entire page has been translated. Alternatively, you could make it so correctly answering questions also increases translations.
Different levels of difficulty
Some people might be just learning French. Basic words are more relevant to switch. For more fluent speakers, having harder words more-frequently highlighted would be great. Maybe make people take a 5 min test on install, or be able to take it anytime to test competency?
There are many interesting compound phrases in French. Would be cool to learn these too. E.g. Flu shot -> Vaccin contre la grippe
It would be great to see the different verb conjugations for a given tense when you hover over a conjugated or infinitive verb.
Better context awareness
Try and show translations fitting the context. For example, if someone says 'religion has been subject to many ..' subject is translated as matière, but this phrase would be 'a été soumis à'. I suggest adding and prioritizing the longer translations