iNaturalist Network nodes are partnerships between iNaturalist and at least one institution in that node’s country. You can read more about the network here and see the network application here. As far as I know, no institution in Japan has sent us an application to become a network member.
iNat is not used too much in Japan. That’s probably due to many reasons but one big reason is likely that neither the website or mobile apps are well translated into Japanese - both having been sitting at 49% translated for a long time now. IMO the first thing to do would be to get the those translated into Japanese - it made quite a difference in Thailand (as well as some groups doing excellent outreach there), and get it to the attention of nature-related institutions in Japan. But there really needs to be a core group of people the institution who know and will do the hard outreach work necessary for a node to be successful.