Programming

Scheme, and work through SICP, watch the lectures along with the reading.

I prefer Chez Scheme but there's many implementations. Chez's fast and practical, C FFI, large standard library, nice REPL with editor.

Perhaps Elixir is worth a look.

Download GODOT and learn the scripting language and the shader language. Make a game! Or at least neat shader art.

Bonus answer: Csound. Make funky noises with code.

I love C#, but F# is also super fun and worth checking out.

Clojure, it's a lisp that runs on the jvm and would be quite the shakeup for ya.

Kotlin would be the most straightforward/useful professionally if you're primarily Java. Other Java stack options are Scala or (shudders) Clojure

As the other person said, Python. Or if you want something lower level, how about Rust?

PHP is a really fun language syntactically and has a surprisingly good built-in library.

Ada and COBOL are still where the big money is, and still will be for years to come.

I think Rust and C# are the future.

Controversial opinion, but I think Python, Java, VB, and others will become legacy languages. They'll be around for 30-60 years, just like Cobol, but I expect things to settle around other languages.

Lua is pretty fun.

Since you already know Java, you could jump straight to C++ with Bjarne's book "Programming - Principles and Practice Using C++": https://www.stroustrup.com/programming.html

You can then move to more modern C++ with his other book "A Tour of C++": https://www.stroustrup.com/tour3.html

And then if you're curious to know how software design is done in modern C++, even if you already know classical design patterns from your Java experience, you should get Klaus Iglberger's book: https://www.oreilly.com/library/view/c-software-design/9781098113155/

In parallel also watch the "Back to Basics" video series by CppCon (see their YouTube channel: https://www.youtube.com/@CppCon , just type "back to basics" in that channel's search bar).

Learning proper C++ should give you a much better understanding of the hardware while the syntax still remains elegant, and you get to add a new skill that's in very high demand.

Odin is quite a fun new language I just started learning. It is meant as a C replacement and comes with a decent standard library and third party library so there is a lot already built that you can use. It also is fully compatible with C and can use C libraries.

Just be warned that documentation is lacking and you will have to read the source code of the standard library from time to time or seek help from their discord.

Seriously - try PHP.

It's a much much nicer language than Java or JavaScript and unlike some of the latest languages (which are, arguably, even nicer) PHP has a massive library of third party packages that you can either use or just learn from.

And unlike Python, which is a general purpose language, PHP is purpose built for exactly the type of work you're doing.

Most PHP websites are wordpress, which deserves all of the crap people ditch on it, but all of that crap is wordpress not PHP, and there are other options. Start with this: https://phptherightway.com/pages/The-Basics.html

C

I love Clojure+Clojurescript

I say that you should find some interesting project, possibly something related to some desktop environment like Gnome, KDE, sway, cosmic and so on. There are multiple fun/interesting projects around them. Then pick a small and manageable task, use that to learn the language that project uses.

I find Cosmic to be a very interesting desktop project, and they use Rust if that would be of interest.

If you want something that feels more quirky, go with Lisp.

While it's related to your job and not very new compared to your current stack, it's very worth it to learn typescript. It has a cool type system and makes frontend development sane.

Factor!

It's incredible and elegant and defies some common categorization.

I've put some of my favorite resources in the sidebar of https://programming.dev/c/concatenative and I'm happy to walk through any particular challenges/examples -- I've done about the first week of Advent of Code with it this year, and the most recent handful of Perl Weekly Challenges, and some basic Euler problems.

DotNet is closest to Java, but hang on to your hat: the state of C# is at least half a decade ahead of Java, if not a full decade. It’s sophistication will make Java use feel like banging rocks together. DotNet Core can now run on all three primary platforms, and with some careful work, you can write a single program that can compile down to each platform and carry along its own required binaries, no pre-install of any framework needed.

My second recommendation would be Rust. Stupidly steep learning curve, but an absolutely game-changing one where safety and security is concerned. It’s my next objective, personally speaking.

Any other language I could recommend starts getting into speciality purposes, which makes general use more difficult or even wholly inappropriate.

For example, if you are dropping into DotNet for business applications, I would also recommend diving into F# for that functional goodness for building complex business rules and data handling. But building an entire app in F# can be jamming a round peg into a square hole under many circumstances, it’s appropriateness envelope does not cover as many cases as C# does. You want to use C# for boilerplate/frameworks, F# for the core bits where it is going to shine.