I would say if you have an appetite for learning something new, then pick something which nicely complements your existing knowledge and broaden your horizon.
For example, if you are a Java/C# corporate developer then maybe learn something different like Go, Rust or go for a complete paradigm shift and look at a functional language which will teach you complete new things and change your overall thinking of software development.
If you have been mostly doing backend why not learn a frontend language? Just pick one which you like the most from your initial gut feeling, don't overthink it.
If you've worked with a lot of dynamically typed or interpreted languages, then pick one which is statically typed and maybe compiled.
Basically, just learn something truly new, which will most certainly teach you something regardless if you will continue doing it for a long time or not.
For example, if you are a Java/C# corporate developer then maybe learn something different like Go, Rust or go for a complete paradigm shift and look at a functional language which will teach you complete new things and change your overall thinking of software development.
If you have been mostly doing backend why not learn a frontend language? Just pick one which you like the most from your initial gut feeling, don't overthink it.
If you've worked with a lot of dynamically typed or interpreted languages, then pick one which is statically typed and maybe compiled.
Basically, just learn something truly new, which will most certainly teach you something regardless if you will continue doing it for a long time or not.