People often ask which is the most difficult language to learn, and it is not easy to answer because there are many factors to take into consideration. Regarding first languages, 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. Some people seem to learn languages without any effort, while others find it very difficult. Besides, people learn languages that they need to use professionally much faster than languages that have no direct use in their everyday life.
Many people answer that Chinese is the hardest language to learn, possibly influenced by the thought of learning the Chinese writing system and by the fact that the pronunciation of Chinese appears to be very difficult for many foreign learners. For example, Japanese speakers, who already use Chinese characters in their own language find learning Chinese writing less difficult than speakers of languages using the Roman alphabet. So, generally, people seem to think that languages which are related to our first language are easier because learning a completely different writing system is a huge challenge.
However, it can be argued that this is not the only factor making learning one language more difficult than learning another. In fact, quite surprisingly, British diplomats and other embassy staff have found Hungarian to be the most problematic language. So, in the end, it doesn't seem possible to say that there is one language which is the hardest language in the world.