MacRuby (which RubyMotion uses on OS X) is a custom version of Ruby with some significant changes under the hood and a lot of compatibility shims:
https://github.com/MacRuby/MacRuby/wiki/How-Does-MacRuby-Wor...
And MacRuby itself is written using Objective-C.
Objective-C (and now Swift) is OS X's system language and they are not applicable as a system language for any other platform.