Go is low-hanging fruit. You can pick it up in a weekend pretty easily. Rust is a much steeper learning curve, but it will also deepen your understanding of type systems and memory management. Julia seems like it hasn't broken out of the data science niche in full yet, and even there it seems like it hasn't made a ton of headway with respect to marketshare--it doesn't seem like it appeals much to me.

If you want something halfway between Go and Rust, I'd look at Gleam--it seems like a garbage collected Rust, so you get a nice ML-inspired type system without the pedantry of the borrow-checker (sometimes that pedantry is helpful, but rarely so for the applications I write). This would be next on my list of languages to learn.

Also, my favorite way to learn a new language is to write a static site generator. It will take you through a tour of the standard library as well as the third-party library ecosystem. It's a simple project without being a toy.