I'd vote for Malbolge, if we're counting esoteric languages. The language was designed with the express purpose of being as difficult to use as possible, going as far as requiring the source code itself to be encoded in a cypher, making it essentially impossible to read. Beyond that, it uses base 3 arithmetic, and essentially requires to use of self modifying code to accomplish most tasks.

There are very few programs written in it, with the first hello world program appearing 2 years after the language was published. And that code was not written by hand, but instead generated using a beam search algorithm.

If we relax the "uselessness" requirements a bit, and focus more on languages that are simply incredibly different from the ones you've learned, there are a lot of potential candidates.

Prolog is a logic programming language where you basically describe the problem and the compiler finds a program that solves it (assuming it has a solution). This is a very radically different way of writing code compared to the languages you have experience with.

Similarly Rocq (previously Coq) is an interactive theorem prover, where you wrote code that describes mathematical logic modeling something and can find formal mathematical proofs about it. It is possible to write programs and verify their correctness in Rocq as demonstrated by the CompCert C compiler.

Koka is a functional programming language with algebraic effect types, which clearly define what kind of side effects (if any) a function is capable of causing.

Haskell is an extremely powerful functional programming language with an absolute crapton of functionality that can be enabled for all sorts of advanced type level programming. As a bonus, since everyone loves to create custom operators, it's not uncommon for Haskell code to look like symbol soup with a handful of recognizable names mixed in here and there.

[APL](https://en.m.wikipedia.org/wiki/APL_(programming_language)) and other array languages treat everything as arrays and provide a bunch of super basic functions to manipulate them, leading to solving problems in a very different way than you might in other languages. As a bonus, APL specifically uses a bunch of custom symbols, leading to code that looks like this:

life ← {⊃1 ⍵ ∨.∧ 3 4 = +/ +⌿ ¯1 0 1 ∘.⊖ ¯1 0 1 ⌽¨ ⊂⍵}

This example implements Conway's Game of Life in APL.

You could also try Racket, as another commenter mentioned, or any other Lisp really, as they are different enough from what you're familiar with to require learning to think differently about solving problems.

There are tons more languages in various different paradigms you could try, and they would all be valuable right now. All the languages you;be learned sit within a very tiny corner of the world of programming languages, and offer only the tiniest glimpse into the variety of languages, paradigms, and ways to think about solving various problems.

While I respect that you aren't interested in learning something just because it's "useful", I think putting emphasis on the "uselessness" of a language is not the best way to think about this. Instead you should consider "how different is this language from what I'm familiar with", because the more different and alien a language feels, the harder it will be to learn, but it will also reshape how you think about solving problems.