Rust but purely because if you only speak JS you'll learn some rewarding new concepts that JS hides from you - both in language theory and how computers actually work under the hood.
Programming languages are to programming what foreplay is to sex - important but not the main exhibit, my point being that Python and (especially Go) are relatively simple languages coming from your background so you could realistically learn all 3 in a few months if you really wanted to.
Programming languages are to programming what foreplay is to sex - important but not the main exhibit, my point being that Python and (especially Go) are relatively simple languages coming from your background so you could realistically learn all 3 in a few months if you really wanted to.