How to switch keyboard layout on right cmd press
Here you go! https://github.com/tekezo/Karabiner

disregard below, only there for history.

~Sadly I did a lot of research on the modifier keys for OS X a while ago, and you cannot differentiate left and right cmd modifiers.~ However the keybind normally for changing keyboard configuration is cmd-control-space for myself. Thats the normal configuration to select the next input source.

This is kind of like the annoying fn modifier that has no use but Functions 1-12. But that can be fixed with software.

Would have posted as comment, but I can't post commends yet. 

edit: Oh, and shortcuts always require two key presses. A modifier and a primary. Otherwise things wouldn't operate quite normally if you understand what I mean. Such as allowing an [a-z] key to be launching an app.