Would it make more to sense to learn more SQL or another language?

SQL doesn't sit in a category that's similar to "other" languages. You often need to know how to construct SQL queries in addition to working in another language.

Honestly, learning a language to "improve" yourself is probably a waste of time. Learning a kind of language is more valuable. Not SQL, but declarative. Not Haskell but functional. Not Java but object oriented.

Additionally, it'll be more useful to understand programming concepts, like SOLID, flow control, data structures, etc.

As a developer for ~15 years, I don't generally fear projects using languages I'm not familiar with anymore. I know I'll pick up the basics to start contributing, and I'll grow into mastery over the ensuing months.

Which brings me to my real advice; learn how to be productive while learning. If you can do that, you'll have a long and successful career as a developer.

Just learn both. SQL is not that big unless you want to go full analyst / dba. Spend some time on it and treat it more like regex - you can look things up if you need it. The this or that choice as you explain it doesn't feel right.