With Mario Kart gone. Splatoon has seized the throne and it is not letting go.
I too, predict 1 million sales in Japan during launch week.

Looks like Splatoon 2 is the new generation of animal crossing in Japan. I can easily see the LTD numbers for splatoon are around 3-5m.

Splatoon 2 - 3-5m seller
Animal crossing switch - 4-6m seller
Pokemon Star - 3-5m seller
Mario Kart 8 deluxe- 1-3m seller
Super smash bros- 2-3m seller
unannounced MH series - 3-4m seller
Dragon quest 11 - 1m seller
Mario odyssey - 1-2m seller

15million switchs are locked in Japan

I do wonder if Switch can outsell 3DS in Japan with the heavy hitters like Splatoon 2. All Nintendo need to do is put some other big games like pokemon and MH on it imo.

Between Splatoon 2, Animal Crossing Switch, Pokemon Gen 8, and most likely Monster Hunter 5, Japan officially The Land of the Switch.

It depends if Nintendo will release multiple versions of the Switch. Like the 3DS has the XL, 2DS and the NEW ones. Lots of people might have got 2 or more 3DS which contributed to the overall sales. 

Zelda is traditionally a pretty front-loaded title! It also has 4 weeks in that first month. We really don't know how front-loaded we should expect Splatoon 2 to be.

Those Mario Kart and Monster Hunter sales are pretty significant! They are two of the most popular franchises in Japan, releasing in the middle of the Holiday season on the most successful console of the generation at a point in its life when it had a 2.5-3 million base. And they did not reach 1 million first-week sales COMBINED. Regardless of what's going on around Splatoon, my expectations are tempered. Can you even give an example of any game within the past 2 generations launching with 1 million first-week sales in Japan within the first year of the launch of its platform?

1st: Zelda is nowhere near the level of popularity Splatoon is in Japan. The best selling Zelda game in Japan, the first one, is at 1.69, on the NES, which ended up with an install base of 19.35 million over there. The first Splatoon is at 1.52 million on an install base that is not even 20% of the Famicom. 

2. The Wii version of Monster Hunter Tri had already been out in Japan for close to 2 and half years at that point. It wasn't an original, made from the ground up, Monster Hunter game like 4 was in 2013, it was a port and because it was a port, it didn't do NEARLY the amount of numbers as expected from a ground up, original Monster Hunter game. As for Mario Kart, it's one of the popular franchises in Japan, but not as popular as Splatoon. I mean the first game managed to outsell Mario Kart 8 on the Wii U over there, didn't it? And Mario Kart 7 finished with close to 300 points on COMG during the holiday season and launched with over 431k. Splatoon 2, so far, is on pace to pass Mario Kart 7's mark by as early as next week. It is going to absolutely OBLITERATE 7 here. I don't remember exactly how many points the first game finished with, but it was practically a 1:1 ratio with launch. So, if the same holds true here, I think the second game is going to do it.

There may not be any games that come to mind that have been able to pull it off before, but you know what? First time for everything! If there's one game that I think could do it, besides Pokemon, it's this one.

That was a sad read.

1m first week sales are not entirely impossible but surely splatoon 2 is gonna get into the 700k-900k territory and that's why a comparison with MHXX is more accurate for launch number of the opening week.

To your first point... but... you were using Zelda's US sales for your comparison. It sold only about 190k its first week in Japan, and that's not bad! It's in-line with traditional sales for the series -- for example, Ocarina 3D sold about 165k its first week in Japan. I still don't see what any of this has to do with Splatoon 2 selling 1 million its first week. Let's wait to see Mario Kart 8D sales. (Edit: I guess you were originally illustrating that it's technically possible for a game to have a 100% or similarly ridiculous attach rate at launch. I'd counter it may have happened with Zelda due to the circumstances being right, especially it being a launch title, but it's a highly situational phenomenon and I wouldn't count on it.)

Downplaying Monster Hunter and Mario Kart doesn't do it for me. New Super Mario Bros 2 sold 425k its first week. Animal Crossing New Leaf sold 640k its first week.

Look, I'm not saying your prediction is wrong, just that it's bold and -- in my opinion -- unwarranted. You're talking about a level of sales right out the gate exclusively reached by the very few titles that are obscenely popular and heavily front-loaded in Japan, AND have the benefit of releasing on a platform with a huge install base. These are Pokemon / Monster Hunter circa 2013 expectations, on a system four months old! The math isn't there for me. It implies that Splatoon is bar none the biggest gaming IP in Japan right now, and maybe you believe that, and that's fine, but it hasn't proven that yet, and I deal in numbers. This would be unprecedented as far as I'm aware. It requires a perfect storm to pull this off (Splatoon has to be as popular as we think it is, AND it has to be front-loaded which has not been tested, AND it has to overcome a relatively small install base partly hindered by stock shortages).