General answer :

In my opinion all domain-specific languages (DSL) are underestimated, because their use has a very narrow niche. Thus- it is used by a small segment of targeted developers and is mostly unknown to mainstream developers and/or to the general public.

Specific case

One example is GLSL. It is mainly used for programming different types of shaders (vertex, fragment, geometry, compute,..etc) which targets OpenGL graphics library for processing data pipeline on video card. The basic reason why GLSL is underestimated in PC development is that video pipeline processing is mostly important in PC video games and these are mostly produced for Windows operating system, which mostly uses other proprietary graphics library DirectX. Which in turn uses it's own proprietary HLSL shading language for video shaders programming. Some Windows games also supports OpenGL library, but these are minority. So GLSL and OpenGL targets mostly Linux operating system's video applications. Of course HLSL is also a type of DSL, thus is also an underrepresented programming language. However I wanted to expose GLSL more, because it's domain is yet more narrow in PC development than HLSL, as market share of computer video games shows.

I think you're looking for "smaller" languages that don't get a lot of attention? ie. not languages like Haskell, Scala, R, etc which have a small share despite being pretty well known.

One that I have seen popping up a lot is Elixir. I also know a lot of people who are super keen on Elm, although most aren't game to actually base their production systems on it (at work we use an Elm-style state system in Typescript).

I'm surprised how many people I know personally who like Python, particularly as a learning language, versus the industry's supposed apathy towards it. I've heard a few people tipping a bit of a Python resurgence due to its popularity in AI/ML/NLP and so on. We'll see, I guess.

I do think people under-estimate how valuable it is to learn scripting languages, particularly Shell/bash and its windows cousin Powershell (while available elsewhere, I'm yet to see PS really take off outside windows). So despite being ubiquitous, they don't get much attention.

Then you get HTML/CSS/JS, which all get attention - but most of it seems to be nonconstructive. So should we count that as "attention" at all? ;)

it's the coder who makes the language legendary :)

I will go for Perl. There is such a huge amount of linux programms still running on perl scripts.

Perl6 has a nice features like github.com/perl6/nqp and finally have multiple virtual machine targets and a nice grammar feature docs.perl6.org/language/grammars also their lowlevel concurrency model improved quite a bit.

But the main point is that we still use their tooling a lot without even knowing that we do.

Python.

I have never seen developers recommending newbies to learn python as first language. They usually preach what is based on stackoverflow crap data. Python is one good language that can make you working on serious stuff pretty quickly compared to other languages.

Recently data science industry adopted it so it's back in light again. But for web development people wasted a lot of time on java and asp.net for years, they could have made frameworks with python instead. I also like Python community because they don't go on hate mode on other languages. Otherwise there seems to be a lot of hate from the language communities like Rust, Go and Web Assembly. It feels good to be part of community where they do their own thing and don't preach the hate.

Apart from python, I'd also say PHP, C# and Ruby don't get much recommendations either. But Job boards show you the truth about real market demand, instead of what people recommend in communities and forums.

I would vote for JavaScript.

I guess each domain has its own unloved, or even unknown, share of languages, which are great in their own right. For most of what I do, I'd say that Reason is one of them. Reason is mixing the best of OCaml and JavaScript, while providing very modern features. I'd love to use it a lot, however the tooling is very very bad and the community small. It is a fairly unknown and unused language, so I don't expect a lot from it at the moment, and I wonder if it will ever fly - which is a shame. Maybe it will, it's a facebook project after all..........

Simple, fast & type safe code that leverages the JavaScript & OCaml ecosystems

type schoolPerson = Teacher | Director | Student(string);

let greeting = person =>
  switch (person) {
  | Teacher => "Hey Professor!"
  | Director => "Hello Director."
  | Student("Richard") => "Still here Ricky?"
  | Student(anyOtherName) => "Hey, " ++ anyOtherName ++ "."
  };

I think to answer this properly, we need to know how to measure popularity.

For example, if you mean a language that isn't used much yet in industry, then I'm going to say Rust (we all saw it coming). It has been the top liked language on Stackoverflow by a margin for several years, but not a lot of companies use it (perhaps because it's fairly new).

Another language that I feel deserves to be much more popular is WebAssembly. But unless you build compilers, you probably never write it yourself. So maybe that's not type of answer you were hoping for.

If instead you mean a language that people hold a bad opinion of but actually has a lot of advantages ("infamous" rather than "unknown"), that's a harder question...

I think some older languages that used to be rather terrible by modern standards - PHP, Javascript, especially Fortran - are better than they used to be. But some things cannot be fixed due to backward compatibility, so I personally still wouldn't call them great...

Going by the most dreaded list from 2019, my initial instinct would be that Ruby is out of place, but I only know a little about the language...

EDIT: Another category is languages that are only used in some domain, so while possibly popular there, many people outside that area don't know about them. For example, I think Swift and Powershell are both quite cool, but if you're on Android/Linux you probably won't see them.