The hard stuff is problem solving, understanding algorithms and their applicability, implementing and using data structures, how computers work, how they represent types and resources, how networks work, network protocols, databases, processes, threading, asynchronous coding etc. All of this knowledge is mostly independent from programming language and platforms and will apply from one language to another. Learning the syntax of a programming language is trivial when compared to other programming topics.

If Ruby and Ruby on Rails dies tomorrow or Python and Django or whatever, and you know of the things I just mentioned picking up the next thing should be not be a difficult task. I don't really care what language someone already knows when it comes to hiring. If you don't know JavaScript, but you know Ruby or Java or C#, and you understand Computer Science, have demonstrated being able to write working code and are not afraid to ask questions picking JavaScript on the job is OK with me. We'll do code reviews, explains things, get you going.

Drats, as I was writing this, someone did:

http://opalrb.org/