That’s a good question 
@mightycraft is a Japanese company and game studio that will help promoting GDevelop and offering Japanese support (through this forum or their company).
As I’m confident they can do a good job, I’ve set up a Japanese forum that they will moderate. They also created a landing page to introduce GDevelop to Japanese users, as well as translating the website and the app
I open forums in languages other than English on a case by case basis - as I can’t moderate them (apart from French). It’s important for me to have a dedicated moderator that I can trust and that have a good commitment in the community and the app.
Usually I’m fine with an external forum that is hosted and owned by someone else - if that’s the case I’m happy to link to it to make it “official” Just as ddabrahim explained.
In the case of the Japanese forum, it was just easier to go and create it here as Mightycraft did not have any forum hosting.