That is a very logical and interesting question. Your need for swift will change as you attempt different areas of the game. To give you an example,
When you start working with Arrays and Strings, you will need to brush up on Arrays, Strings and related things like String.Index, converting strings, and functional like map, reduce, filter, etc.
SpriteKit can help you with the UI side of things, but the glue holding it all together would be swift, you will then also have the need to save data, which can be using strings/file writes or decoding it as JSON or adopting other ways to manage the same.
To have a good smooth gameplay you will require to know about threading, so … back to the point, you will learn as you will require it.
If you want, start with a simple project and you can learn as you go and you can find lots of articles, etc that would help you overcome the hurdle that you might face.
All the best, and I’d personally keep a look out for what you make.
There is a really good tutorial on SpriteKit on Udemy, from Brian Advent. I’d say if you work through the Swift Apprentice (rw plug) or even the Big Nerd Ranch Swift Programming book and do the challenges, you’ll have a good foundation for getting into SpriteKit. You CAN do spritekit without some of the data structures, but you won’t get far before you realize you’ll need some of those basics for typical UIKit components. I really suggest working through the a basic swift book before diving into the sprite kit stuff. Also Ray Wenderlich has a good 2d iOS Sprite Kit book… linking (2D Apple Games by Tutorials)… I’m through the cat lady part of the book but still working the rest as I have time. I don’t think I’d be doing as well without some of the foundations of other Swift and iOS basics though.