Splatoon 2 is immensely popular in Japan. So popular in fact that residents build working models of the in-game paint guns, and host multiple events including holographic Callie & Marie concerts. The first Splatoon did very well for itself over there, but the low popularity of the Wii U didn’t help things.
Now with the Switch to assist in spreading its wings, Splatoon 2 has soared over most of the Japanese games industry. According to Media Create, Splatoon 2 has sold two million copies just in Japan. This figure might not seem like much given that so many games like GTA V break records in the west, but over there it doesn’t happen all that often on consoles. (Portables are a different story, which have hosted several multi-million sellers.)
The last time that number was reached on a console in Japan was nearly a decade ago, back in 2010 with Wii Party (love that game!).
Splatoon 2 is first game to hit 2 million sales in Japan this decade [Metro UK]