People often debate what is the hardest language in the world to learn. Assuming your native language is English or one of the other Indo-European languages like Spanish, German, Russian, or Farsi, then an Asian language like Chinese, Japanese, or Korean is an obvious choice. In other words, the language you start off with affects which languages are easier to learn next.
If you are a native Spanish speaker, you’ll have little difficulty learning the other Romance languages (French, Italian, Portuguese, etc.). If German or English is your first language, then the other Germanic languages (Danish, Dutch, etc.) go easily, and the Slavic and Scandinavian languages aren’t too much of a stretch.
If Japanese is your native language, then it is easy, and Korean isn’t too bad. But otherwise there’s good argument for saying that Japanese is the hardest language in the world, with Chinese being a close second.
Japanese is hardest becaus, first and foremost, the writing system is catastrophically confusing and convoluted. My classical Japanese history professor said that the worst thing ever to happen to Japan in its entire history was the introduction of Chinese characters. The Japanese took them, gave each several readings (pronunciations) and sometimes as many as two dozen, then created two syllabaries (like alphabets, but larger), and over the next 1,000 years came up with all sorts of bizarre, inconsistent rules about when to use characters versus the two syllabaries, and how to combine them to write the language. After the Meiji Restoration, the Japanese decided that their writing system wasn’t complicated enough, and so added the Roman alphabet for writing certain words, and for the convenience of foreigners.
Now Chinese does use more characters than Japanese does, about five to six thousand compared to the two to three thousand in daily use in Japan. So learning to read Chinese does require learning a lot more characters. However, since each character is pronounced in only one (or sometimes two) ways, what has to be learned for each character is much simpler.
There are no rules about when and how to combine characters in Chinese with anything else, since that doesn't happen. Japanese, by contrast, has complex rules about when and where the hiragana shows up with characters. These rules are sufficiently troublesome to challenge many native speakers. Then there are personal and place names, which are given readings almost at random, often completely unrelated to the standard pronunciation for the character.
None of this is an issue in Korean, which has what is arguably the most brilliant, efficient, and effective writing system ever developed in the world. Learning to read and write Korean does take effort, but only about as much as learning the Arabic script when learning Arabic, Farsi, or Urdu, or learning Devanagari for Hindi.
Grammar in Japanese is a bit messy, with verbs doing all manner of bizarre things, adjectives behaving exactly like verbs at times, and all of this having to be changed depending on who is saying what to whom under what circumstances (what is called “respect language” in Japanese).
Korean has “respect language” too, and it’s even more convoluted than Japanese, but the grammar is otherwise comparable. Chinese grammar is remarkably simple and efficient. Nothing changes ever (no conjugating verbs, declining nouns or adjectives, no gender or number for nouns, no nothing).
Finally, pronunciation. Chinese is definitely the most challenging of the three because of the tones. Japanese and Korean are not tonal languages, so pronunciation is relatively straight-forward.
Most Americans and Europeans who learn Japanese (or Korean) struggle to purify and clean up their vowels and consonants but manage well enough.
Chinese, by contrast, is very difficult to learn to pronounce, though the Chinese themselves are quite forgiving and helpful about tones, so the process can be pleasant and even fun.
So there’s my take on a question that I hear a lot. It is obviously debatable. And I am not trying to convince anyone that Arabic or Tamil, or Finnish or Hungarian, are easy languages to learn, especially if your native language is English. But I think that, unless your native language is Korean, Japanese poses so many challenges that it is arguably the hardest language in the world to learn for non-native speakers.