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.