>I've seen a big push from Fast.ai for Swift (they claim it's the future, etc)

You've seen some dated stuff, from before S4TF became dead in the water.

The indisputable most useful language for ML is Python. The ecosystem is by far the strongest, and the language more-or-less stays out of your way while you interact with specific libraries that do what you want. Those libraries, are written in highly optimised compiled languages like C/C++, so are extremely efficient. As long as you keep them fed, you'll see very little of the "python-slow-interpreted-bad".