I don't think it matters which language you learn, learning a language unlocks your brain in a certain way to help you in all sorts of things! It is a very good way to use and train your brain. There is lots of research into the benefits of being bilingual that are far more important than employability. I think you should choose a language you will be able to assist and brush up on because nothing helps in learning than having someone to practise with.

I am fluent in German and I've basically never used it besides when traveling in Germany but I know it was an invaluable tool to learn and would never think of it as a waste to have done.

ETA: I do think it is really important to have someone to keep your skills near the surface, I lost so much of my ability just from having no reason to use it. I then made a German friend so that keeps it relevant and forces my brain to retain it all haha although I know I used to know things a lot more or faster mostly... I did learn about 15yrs ago now... when I was young, about 7 I think? Until year 10 so many, many years of learning.