I find it easier to learn a language if the text to speech voice that I'm listening to is male. Men and Women speak differently enough in pitch that I sometimes struggle to speak words which contain unfamiliar sounds with only a Female voice as reference. This may sound like a minor complaint or, worse, be seen as some kind of politically incorrect misogyny. But, this issue causes cognitive friction for users and increases the time & difficulty of language learning. Because of this, I consider this a very real pain point that has a relatively simple fix.
Filter words translated by type
It would be cool to be able to filter what kind of words I get to practice. For example, I am learning French and one week I might want to only be learning verbs and another week only adjectives.
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.
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.