A very common question among people who are tinkering with the idea of studying a new language is What is the hardest language to learn? As I wrote in response to the question What is the easiest language to learn, the difficulty of a language depends largely on you and what your native language is.
Difference From Your Native Language
If your native language is English click here. If your native language is Korean then Japanese is definitely not the hardest language, because it is so grammatically and conceptually similar to Korean. If you speak French, then Spanish is definitely not the hardest language because the two languages are closely related and members of the same Romance language family. Basically, the more different the language is from your own, the harder it will be.
Difference From Other Languages You Know
If you already speak a language other than your native language, then a language related to that one probably wouldn`t be the most difficult for you. A language with a lot of concepts and linugistic features that you have never encountered before would be more difficult.
Complexity of the Language
The complexity of the language is another factor. Some languages require more vocabulary for daily functioning, some languages require you to make lots of syntactical changes in different contexts, some require you to make phonological changes in different contents, some languages have very distinct registers, and some languages have very complex writing systems. It`s hard to say which language is objectively the most complex, and I may be biased as a native speaker, but two examples of complex languages are Japanese and Arabic.
Japanese has a complicated writing system using four different types of characters including ideographic kanji (Chinese characters). Even if you are only interested in learning to speak the language and not read it, it is very difficult to be fluent in Japanese without knowing kanji because they form a large amount of the vocabulary and many words composed of kanji sound exactly the same. You distinguish such homonyms by mentally referencing the kanji you know. And kanji are not always pronounced the same way, it depends on the context, with some kanji having 5 or more different “readings”.
Japanese vocabulary is also very complex because there are often numerous distinct compound words which correspond to a single word in many other languages. This is partly because of the use of kanji to form compound words, whereas in another language like English you would probably use one main word and add a modifier to change the meaning slightly. Japanese culture is also very different from most others in the world, and the culture is a big part of the language, so the way of communicating ideas in Japanese can be very different than non-Japanese people are used to. But it`s Kanji that really makes learning Japanese a time-consuming challenge for many people.
Arabic is often considered a complex language because of its phonology which contains some sounds rarely found outside of the Semitic language family, and because of its mysterious cursive alphabet. In reality those are not what makes Arabic complicated. What makes it so complicated is diglossia, using two different versions of the language for different purposes. Modern Standard Arabic is the official language of Arabic countries, and is virtually identical to the language of the Muslim holy book the Qur`an, which was written around 1500 years ago. But the Arabic spoken in daily life has evolved quite a lot over those 1500 years, meaning that second language learners have to learn two different languages: Modern Standard Arabic, plus a spoken dialect. There is great variation amongst dialects as well, meaning that second language learners also need to learn strategies of blending Modern Standard Arabic with their dialect of choice in order to communicate both naturally but also in a way that`s understandable for speakers of other dialects. To learn a single Arabic dialect is not really complex. But to become fluent in Arabic in a way that you can communicate with speakers from various countries, takes an impressive amount of time and effort.
The Various Factors
Which language would be the hardest for you to learn, depends on how different the language is from your native language and other languages you know, as well as the complexity of the language. There is no definite answer that applies to all learners.
See also: What is the Easiest Language to Learn For English Speakers? (this also discusses the hardest)