I frequently have an empty popup with just a star and audio option, no translation that's sticky and remains when I scroll away
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
Higher contrast options for the highlights
Not just for accessibility, but having the words pop out more may help you better remember them.
Hear me out… Fluent on iPhone
It’d be nice to be able to track your progress and see words that you’ve learned from your phone and their definition, use cases etc... It could also have notifications for unfamiliar words you need to practice.
Gradually translate all words
It seems that when I click "I know this word" it doesn't translate it again the next time. But when I know a word, I want it to translate every time, with the goal that eventually all words will be translated to French, so I'm reading the entire article in French, except for the words I haven't learned yet -- which will gradually be highlighted and translated.
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?