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.
I'm more of a web dev "code monkey" and good enough writing simple backend apps.
I was actually thinking about more deep-dive into database design and what`s not. I am fairly sure I can construct queries and understand simpler topics like indexes. However, I am completely lost on optimization techniques like sharding, for example.