It's interesting. I love the idea. Some of code snippets can look larger. Look at the http://rosetta.alhur.es/compare/Java/Python/#Call_a_function for example. Java code has missed function definition (definition of class and method inside it) in comparison to Python code. It's unfair a bit! Not because I'm Python nerd :) But because of it too.

Additionally I would suggest to redirect user from http://rosetta.alhur.es/compare/ to http://rosetta.alhur.es/