Ask HN: Which programming languages should my co-founder be great at?

I know, I should have learned how to code earlier! But Good news! I've started learning C++ recently, because I do agree with the saying that if you knew how to code, you would translate the vision of your idea/product better.

However, I have this kick ass idea and I need to launch soon, I plan to get a technical co-founder on board to work on the web development. I have to choose from a list I made and I am wondering which programming languages this guy has to be great at? Java script, Ruby and rails ... etc? Are there any criteria I should be paying attention to?

We'll need to develop a online platform that connects 2 customer segments (multi-sided platform).

Also, please do you have any tips or good cases practices on what I am about to do? (recruiting a co-founder)

Thanks a lot