I'm Indonesian and speak English. Now I'm learning Korean (for these 3 years) and French (for these 3 months). Most of Indonesians find that English is hard. Student learn English for more than 10 years but yet they having trouble using it. From my experience of learning languages and observing, it's a matter of MOTIVATION and ATTITUDE that concerns to how a language could be defined as a hard or easy one.
What's the different from (let's say) Indonesian people who learn English for years and still can't talk much than American who learn Mandarin, that is come from completely different roots, for three months and yet could have an everyday conversation in any situation with natives?
It's nothing else but MOTIVATION and ATTITUDE. Motivation is how one could maintain their interest and energy to keep on going to learn the target language and achieve their goals, while attitude is how one perceive and act to be able to reach their goals in time and effectively, this goes a method and any other things that of how people treat/tame the language right so it's become easier and keep interesting to them over time.
So in that case, there is no hard or easy language. But if you see it from one's native language, the hard language would be some language that has unrelated and different roots, like Mandarin is hard for Francophone, English is hard for Korean, Arabic is hard for Russian, etc.
As for me, the hardest language up until now is English, because it took me more than 10 years just to get me use the language in confidence. And the second hardest language for me is Korean. It is completely different structure, vocabulary, grammar, pronounciation, etc. While I took French quite fast as it has a really common words and grammar rule to English.