Written Chinese and Japanese are the hardest written languages in the world hands down. While other languages have a property of being what's called "phonetic" Chinese and Japanese kanji lack that, meaning that the symbols they use don't give any clue whatsoever as to how the written word is pronounced.
It's extremely simple. English speakers only have to memorize the meaning and spelling of a word, Chinese speakers have to memorize the meaning, symbol and sound. A native English speaker can expect to be completely proficient in its written word by the age of 14, native Chinese speakers pretty much never become completely proficient in their written language. They'll admit that in just reading the newspaper or news there's almost always several words they have to look up.
It's also what contributes to differences in intelligence. You see your brain undergoes a "crystallization" process as you age, unused or unnecessary neural networks get pruned, this reduces your fluid intelligence, yet it's what allows you to increase your depth of knowledge of a subject. High fluid intelligence is what allows you to take analysis, linear algebra and abstract algebra at the same time, crystallized intelligence is what allows you to understand the most advanced mathematical research. The difficulty of the Chinese language means that only people who undergo less of this "crystallization" can become proficient in it, for some reason the way the Chinese language is constructed favors high fluid intelligence. This sounds good, but both crystallized and fluid intelligence compete for the same brain space, so we would expect Chinese speakers to be deficient in their ability to comprehend advanced/specialized knowledge.
Also fluid ability is dependent on neural apoptosis, the programmatic destruction of neural cells. You'll notice many genetic diseases found in Ashkenazi Jews have been found to increase neural apoptosis, BRCA1, Fanconi anema, and LRRK2 induced parkinson's.
