What is the Hardest Language to learn?

I need some help solving a debate. We(me and my friends) need to know what the hardest language in the world is. It has to be learnt from a foreign tounge(like you can't say english is easy as you started at the age of one if your native tounge is english). Any refrences, studies, websites, books etc are welcomed and encouraged.

I want to know what is the hardest language from someone who doesn't speak English or any common languages, thanks for the input so far.