It seems automatic Japanese translation is not supported at this time.
Also, please find a way so that it could work in an existing article, meaning it can be used to add content to an existing article.
Topic on Talk:Content translation
Hello @Narutolovehinata5, Thank you for using Content Translation. Currently, we use Apertium for machine translation support and at this time Japanese is not supported. We will make sure to include it whenever Apertium makes it available.
For adding content to an existing article, we already have a ticket in our development board but we don't have an exact time by when we will be able to support this feature. Thank you.
Hello @Narutolovehinata5 we now have a new machine translation system - Yandex that can be used for translating into Japanese. You can see it here: https://translate.yandex.com/ . We are hoping this will be helpful for users of the Japanese Wikipedia. What do you think? Thanks.