Something to consider as well is learning both. Swift is certainly the best choice for making macOS/iOS GUIs. Other languages are probably better than Swift for your ML needs (could be rust, Python, etc.). However it’s totally possible to have an app using multiple languages. You could have the UI portion be in Swift, but the ML portions be in another language.

At my company we have a Mac app with the GUI written in Swift, shared logic with our Windows app written in C++, and some libraries written in Rust. So it’s certainly possible.

One caveat is that some languages don’t work with each other very well. Swift and Python do work well together iirc, so doing UI code in Swift and ML code in Python may not be a bad idea.

If you want to just stick to Swift, Apple does have some ML frameworks for Swift that you can use. I don’t do any work with ML, so I have no idea if these frameworks are any good, or have good resources for learning.

If you want to just stick with whatever language you use for ML, there are GUI libraries in nearly every language. These certainly won’t be as robust or as nice to work with as the native frameworks in Swift, but they could probably get the job done. I do know that a major issue with GUIs in Python is the difficulty in multi threading, which is a must for any app that performs long tasks without the UI freezing.