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. :)