+1 on the approach @bibliothecary linked!
Some books have terrible writing, unnatural writing style, etc. You could make an argument about those I suppose
If it has a Bookwalker link, that will often contain that info
That said they’re not always 100% perfect, and really those labels are overly broad anyway
Tho that requires things being sufficiently tagged
Paying attention for a friend 
I have an idea actually. What if giving a rating for “language learning” was optional and there was also an optional second component to a review which is something like “aspects that are good for language learning” and then the user could optionally write what they think they got out of the book. Like having furigana if that is the case, or having a lot of natural spoken language, having regional dialect, having easy or difficult vocabulary, etc
Personally I try to review what I read, but I’m not always super diligent with it. It does take me some energy to write them, and I often forget as well to write it right after I write a book as finished, so what ends up happening is that I’ll write a bunch of reviews in a row. I would love to have an option to make it so that when I mark a book as finished it brings me directly to review it (maybe a toggle, so that those who want it can have it, but it doesn’t bother anyone else?).
That being said though, I love reading reviews. Beyond ratings, which I don’t care much about unless they’re super low (and what those really tell me is that I need to check the reviews and see what’s up with this book), reviews really are useful to get a better idea of the level of the book, the writing style, and of whether or not I might like the book. Even just reading reviews that pop up in the general feed is interesting, as I’m always up for reading more books reviews. I don’t particularly care how they’re structured either, as long as I can understand them
In my experience I wasn’t able to upload a review without filling those out
It insists on an overall rating, but the other two are optional: I never fill them in.
Perhaps this might be something for your friends
Is… Is this a villainess romance fantasy that has an audiobook narrated by a man?
Am I dreaming?
