Find out which sort of language features you find really cool that apply to your project. For example I have this idea in my head of writing ML stuff in Haskell

Find out which sort of language features you find really cool that apply to your project. For example I have this idea in my head of writing ML stuff in Haskell, since everything is lazy and you could run models off disk. That would be really fun not only because of the end goal but because writing Haskell code is a borderline psychedelic experience. Using core language features tends to give you that feeling of using divine tools as opposed to writing a textbook in a second tongue. Other than that I'd recommend C. It's ubiquitous and can be as simple as you want it to be. With macros or source to source transformation you can have your cake and eat it too by picking how you want your code to be written then figuring out how it would compile to C.