Comment by GaggiX

Mastodon has massive Japanese communities on it, Pawoo (social that was owned by Pixiv, was the largest Mastodon instance before Musk took over Twitter, now it's second), mstdn.jp (used to be second, now third), now there's also Misskey, which is not a Mastodon instance, but it's perfectly compatible with it.

Mastodon was battle-tested by the Japanese, almost no one used it seriously before them.