There are many interesting compound phrases in French. Would be cool to learn these too. E.g. Flu shot -> Vaccin contre la grippe
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?
Higher contrast options for the highlights
Not just for accessibility, but having the words pop out more may help you better remember them.
Submit corrections to mistranslated words
Would be great to be able to not only "Report" an error but to also submit a correction for it!
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.
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.
It would be great to see the different verb conjugations for a given tense when you hover over a conjugated or infinitive verb.