If you doubt the power and popularity of Splatoon 2 in Japan, doubt no longer. According to Media Create (via NeoGAF) sales numbers, Splatoon 2 sold 648,085 copies during the week of July 17 to July 23, 2017. This is amazing, especially when you consider that the game launched on July 21 and Media Create numbers do not include digital sales.
It also puts Splatoon 2 ahead of Mario Kart 8 Deluxe's sales in Japan. The kart racer is at 588,028 sold to date in Japan according to Media Create, but the official numbers from Nintendo (including digital) had shipments at 690,000 units in Japan as of the end of June 30, 2017. It's likely that Splatoon 2 has a significant amount of online sales already.
The Nintendo Switch itself was boosted to 98,999 systems sold during the week, up from 31,906 units the previous week. This brings the lifetime total for the Switch in Japan to 1,196,986 systems sold. In contrast, the PlayStation 4 just crossed 5 million systems sold within the region.
The original Splatoon launched on Wii U to sales of 144,818 units in the region, but experienced a long tail of consistent sales, leading to a lifetime total of 1.5 million copies in Japan alone.