I would be more concerned that you spent the last X years only using one language, no matter how esoteric it is. If I had to hire someone who has not used the main language(s) in my company's stack, I would prefer someone who spent the last X years working in multiple languages, who should be more likely to learn our language(s) quickly.

From a career perspective, working in one single language seems far more limiting than focusing on some esoteric language(s). I don't even understand how it's possible to spend five years in one language. On a day to day basis I use JavaScript, bash, and python.

Focusing on only C++ or Java or one of the "mainstream" languages seems just as "bad" as focusing on one esoteric language. Try to pick a job where you are exposed to multiple languages/paradigms on a daily basis.

I was extremely worried when I started reading your comment - I've been working on python since I got out of university. Reading the rest of the comment gave me relief - I also used Javascript, PostgreSQL, MySQL, CSS, Sass, Less, React, Angular, bash, Lua, C, GLSL. Phew.

I'd say yes and no. Given that way that a lot of HR departments just seem to run a keyword search for jobs and this often hits languages. This can work against you. If the cool stuff is relevant then that will be good once you get past the HR, if not it's cool anyway. However, unpopular languages can work in your favour when the number of developers drop or the languages become more popular, for instance Cobol was suddenly a goldmine around Y2K and who knows where current tech will go. However, getting extra languages under your belt is a good thing both for employability and learning new programming paradigms.

It's a culture thing.

In your cliche NET 2.x MS-only Windows-Only enterprise setting they won't look down on it, but they might feel you don't have relevant experience.

In many other places people value developers that have non-mainstream development experience because it shows you're willing to explore as a developer and won't shackle yourself to one solution to problems the business faces.

There will be a mix of bias and enlightenment. Much like the rest of life. The exact mixture isn't clear.

You could go work at Jane Street, I think they use Ocaml. https://blogs.janestreet.com/category/ocaml/

To some extent. I think I value what you've done with said tools more than the tools themselves.

You could always spin Clojure as JVM experience, F# as .NET etc.