It really depends on what sort of games you are trying to build. If you want to use SpriteKit or SceneKit (both of which are iOS only), you will probably need t

It really depends on what sort of games you are trying to build. If you want to use SpriteKit or SceneKit (both of which are iOS only), you will probably need to learn Swift. But it's possible you might want to use a cross-platform third-party engine like Unity or Cocos2D, in which case you'd be doing most of your work in C# (for the former), or C++/Objective-C (for the latter). I don't know what the state of game development support for RubyMotion is, though, so take my advice with a grain of salt.