Primarily working on bringing the Swift programming language to FreeBSD, and on the side developing my own systems language, Ripe, and working on the esolang Amnesia in Haskell.

Rust, Odin, Ada, Elixir, F#, Zig, D, Prolog (used once in college), Racket, Elm, Crystal, Common Lisp, Julia, Ruby, Lua, JavaScript, TypeScript, Kotlin, Scala, Smalltalk, Perl, C#, Java, Carbon, V, Scheme, Tcl, Raku

• Language Design Notes
• r/programminglanguages
• The Dragon Book
• Crafting Compilers
• Programming Zoo
• Progopedia
• Rosetta Code

This is by no means an exhaustive list but it's some of the sources that I have used. I am both someone who enjoys implementing and learning about the different semantics of a langauge.

Anyone can submit a PR. If you want to help me in this journey email me, I'll add you to the organization.