My japanese is terrible, but Atsumaru means Gather, Assemble. Probably KADOKAWA is outsourcing job for making DLC's for external company or (and) freelancers. Or - most likely - I found an article about website RPG Atsumaru, where people who make games with RPG Maker and upload them there. But I can be mistaken in both things I said.
If english version of MV don't get updated as Japanese then I will lose all respect I have for this company.
I'm not sure exactly what the Japanese version offers for this, but the English version of the editor never had an option to upload to the Atsumāru site, so I think it's irrelevant for us. Dedicated-language sharing sites can be helpful for native speakers of less widely-spoken languages like Japanese.
If you want, then I don't believe there's anything stopping you from downloading the JP version of the editor, e.g. in Steam: Library -> RPG Maker MV -> (right-click) -> Properties -> Language. You might also need to change to, and/or install, a JP system language pack beforehand to avoid errors.
Trying to get the mechanics down for a cute, story-driven board game! I think I've accomplished dice movement, item effects, and activating a space's event when you land on it. Art is WIP. Hope to open a thread about the game soon!