I wouldn't design an interview just for a specific programming language. If they can prove to me that they understand OOP and functional programming concepts up

I wouldn't design an interview just for a specific programming language. If they can prove to me that they understand OOP and functional programming concepts up to my standards, then they should be able to write useful code given a week or two to learn the language at hand. In the case of Ruby, this time line could potentially be shorter. The syntax is dead simple and the API is rather intuitive.