Looks like there is no Japanese version of wiki logo File:Logo_ja.png yet. Most of other active language already have translated logo, they looks like File:Logo_zh-hans.png this. If there is Japanese version it will be displayed at the top left corner of every Japanese page in this wiki. I wasn't able to find an active Japanese translator in last few month, but If you can provide an accurate translation of “Official Wiki” in Japanese to put in the logo, I can make the image :D.
Topic on User talk:Cicada88
Thanks for your message! I was also wondering about the logo, so that is contact I very much appreciate. Japanese translation of the "Official Wiki" is as follows
公式 Wiki
Thank you for your cooperation!! It was very helpful!