With rejections going out there are lots of people reflecting on where their application went wrong, etc. I've noticed one theme through a number of comments here though: I have my N-whatever, I had 1000 hours of Japanese study, I aced the Japanese portion of the interview…
The problem with these comments is JET doesn't care if you know Japanese. Japanese ability is explicitly not a requirement of the program and you are outright told during the interview (at least I was) that your performance in the Japanese test during the interview has zero influence on your application. This test is for reference because some placements may request a person with a certain level of Japanese skill and they need to confirm what you declared on your paper application, to help you with being placed after being accepted.
Speaking Japanese will be an invaluable skill if you get accepted, and my first piece of advice to new people is always 'start studying Japanese yesterday,' but it isn't something that makes you a better applicant. Your schools and teachers will be very happy if you speak Japanese, but the perspective of JET, it's just as well you don't speak any Japanese because there's no risk you'll use it instead of your native language with your students.
If you were rejected and are trying to improve your candidacy ahead of another application in the future, improving your Japanese ability isn't going to do anything to make you more likely to be accepted.
by an-actual-communism