If you were applying as a CIR, then of course using Japanese would be not only welcomed, but expected of you. But as an ALT applicant, don't use it unless they ask you. If you've written in your application that you know some Japanese, chances are they'll test you on it. And when I say "chances are", note that there are a few instances where they don't test people, despite them having listed that they speak Japanese.
My personal theory as to where your Japanese ability will affect you is when they (CLAIR and the BOE's) consider whether they want to send you to teach at elementary vs junior high or high school. Where I'm placed, you definitely need a good understanding of Japanese to teach at shogakko (elementary school).