To determine whether a language is easy or difficult to acquire, it depends on the learner's first language and the second language he is trying to learn. For a second language that has a different writing system (orthography) or share different cognates from the learner's first language would be more challenging to acquire. For example, Mandarin Chinese might be easier for Japanese or Korean learners to acquire while it's difficult for English speakers to learn. German is easier for English speakers to learn while it's more difficult for Chinese to learn due to the different writing system and grammatical structure between Chinese and German. Also, some people are more gifted with language learning while others have their gift and talent lie in other areas. There are a lot of things involved in second language acquisition. If you are interested in looking into them, I recommend you to read the book "Second Language Acquisition: An Introductory Course" by Susan M. Gass and Larry Selinker. In the book, you will find a lot of interesting theories in relation to second language acquisition. :)

Those which use different alphabet from English letters or Chinese characters are most difficult for me. But at the same time Chinese and English are always the most difficult two for me, cause there is always so much to learn about. From another point of view, the less chance you can get for practicing, the less motivation you have for learning, the more difficult the language is for you.

English is really hard language for us to master. I think that Japanese is easy for learning speaking and listening but hard for learning writing and reading. Russian was very tough to study they say, however, in my opinion, not so much difficult than studying English for Japanese native speakers.

I think , it would be different for everybody , because a few people are able to speak many languages easily and we can tell about then that they're polyglot. We often have a different experience and then we never can tell the same.

The answer is: none of them. There's no hard language in the world. It depends on too many factors to univocally say that there's the hardest language. For example, for French people another language can be hard comparing to Sweds for example. And despite, if we're talking about it, usually only some parts of a language can be considered as hard. Like cases for example.