People often ask which is the most difficult language to learn, and it is not easy to answer beacause there are many factors to take into consideration.
Firstly, in a first language the differences are unimportant as people learn their mother tongue naturally, so the question of how hard a language is to learn is only relevant when learning a second language.
A native speaker of Spanish, for example, will find Portuguese much easier to learn than a native speaker of Chinese, for example, because Portuguese is very similar to Spanish, while Chinese is very different, so first language can affect learning a second language. The greater the differences between the second language and our first, the harder it will be for most people to learn. Many people answer that Chinese is the hardest language to learn, possibly influenced by the thought of learning the Chinese writing system, and the pronunciation of Chinese does appear to be very difficult for many foreign learners. However, for Japanese speakers, who already use Chinese characters in their own language, learning writing will be less difficult than for speakers of languages using the Roman alphabet.
Some people seem to learn languages readily, while others find it very difficult. Teachers and the circumstances in which the language is learned also play an important role, as well as each learner's motivation for learning. If people learn a language because they need to use it professionally, they often learn it faster than people studying a language that has no direct use in their day to day life.