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.