There is no really a hardest language, because depending on where you come from you will easily learn it, or not.
I found French really easy, because I live in Spain and speak Spanish and Catalan, which are directly related to French, whereas I couldn't cope with Russian. However, someone from Ukraine would learn Russian more easily than French! Same goes for any Asian language - surely for someone from Korea Chinese is not that difficult.
You mention English - that is because EFL teaching is not good outside Universities, and anyway to be fluent in English you need to go to England, which is something not everyone can afford.