But she's fine at cooking, the ultimate "look, a flaw" option.
I don't feel like I have to defend Nanase being really good at a lot of stuff, but that doesn't mean I'm not going to have a bit of fun with it.
Writer of Two
I've recently decided Nanase is much better at speaking and understanding Japanese than writing it due to most of her exposure to the language being verbal, and it not being a part of her regular education.
There is a serious commitment to learning enough written Japanese to effectively know "enough". The common standard seems to be "enough to read a Japanese newspaper" (it's an example I've heard again and again), which is over 2,000 written characters, most of which have multiple readings, and that's before using those characters with actual words.
The public schools Nanase went to would not have covered any of that, and it would have been difficult to keep up with her regular studies and do that.
Not impossible, mind you, and probably easier than ever now with modern learning options (though still difficult).
I just think with Nanase specifically, it makes sense that she didn't. It's not unheard of to be able to speak and understand a language (or even JUST understand it) without being able to write it.