Mandarin Chinese, Korean, and Japanese are considered among the most difficult languages to learn for English speakers according to the Foreign Service Institute at the US Department of State. These three languages constitute (along with Arabic and Cantonese Chinese) Category V, being “languages which are exceptionally difficult for native English speakers”. Among those, Japanese gets an asterisk, meaning “more difficult than other languages in Category V”. According to the FSI, they require 2200 hours of study to reach General Professional Proficiency, four times as much as the 575 hours required for most Romance, Germanic and Scandinavian languages. Moreover, Mandarin Chinese, Korean and Japanese are completely unrelated languages. Mandarin is a Sino-Tibetan language along with Cantonese and other Chinese languages, as well as Burmese and Tibetan. Linguists used to group Korean and Japanese into a common “Altaic” family but that hypothesis has been widely rejected, and both language are now considered isolates. The three languages also have very different writing systems : Chinese uses tens of thousands of characters called Hanzi, which all have a specific meaning and sound. Korean has its own alphabet called Hangeul. And Japanese has two syllabaries (Katakana and Hiragana) but also uses a significant number of Chinese characters (called Kanji in Japanese). It sounds like a strange idea to try and learn all three languages at the same time… But actually it’s not such a strange idea. There is some logic in it, because the three languages have a lot in common. Historically, Korea and Japan were very much influenced by the intellectual and cultural powerhouse that was China, and borrowed many concepts and words from the Chinese language. This can help your learning for writing, vocabulary, and grammar.
First, all three languages use Chinese characters to some extent. In the PRC, Mandarin is written using a simplified form of the traditional characters. Japan and Korea normally use the traditional form of Chinese characters in addition to their national alphabets, but each language has certain specific ways of writing them. Korean characters are called Hanja. Even though they are rarely used nowadays, they can still be found in dictionaries and newspapers, and they are very useful for etymology and vocabulary (more on this below). Japanese makes extensive use of Kanji, to write both Sino-Japanese and native Japanese words. Familiarity with one system among Hanzi, Hanja, or Kanji is extremely helpful when you want to learn the other two.
In terms of vocabulary, it is estimated than 60% of Korean words and Japanese words are of Chinese origin (Sino-Korean and Sino-Japanese vocabulary, respectively). However, the percentage of Chinese-origin words used in everyday speech is somewhat lower (estimated at 18% in Japanese). These words sound a bit different in each language, but they are often written the same way in Chinese characters (Hanzi, Kanji, Hanja). In particular, Japanese and Korean form many words with a word of Chinese origin followed by the verb “to do” (する suru in Japanese, 하다hada in Korean). For example, 準備 (pinyin: zhǔnbèi) means “to prepare” in Mandarin (traditional characters), 準備する(junbi suru) means “to prepare” in Japanese, 준비하다(junbi hada, Hanja: 準備하다) means “to prepare” in Korean. Thus, when you learn a word in one of the languages, you can often use it in the other languages as well.
Also, Korean and Japanese grammars are very similar. Both languages use the Subject-Object-Verb word order, both use particles to indicate grammatical functions, both use postpositions for circ*mstances, both conjugate adjectives (pretty much) like verbs. Chinese grammar is very different, and much simpler : word order is Subject-Verb-Object, there are no function markers and no conjugations. Some features apply to all three languages : for example numerals are used with counters which are specific to the type of object being counted. Thus, 本 counts books in Chinese and in Korean (본), and long, cylindrical objects in Japanese. Also, the genitive is expressed with a particle between the owner and the object (の in Japanese, 의 in Korean, 的 in Mandarin), a bit like the Saxon genitive in English and German. Interestingly, attributive adjectives in Chinese also take the 的, while they are conjugated in Korean and Japanese making them a kind of relative clause with an attributive verb (relative clauses in Chinese take the 的, which is kind of consistent !). All this is fascinating stuff, and it can helps transferring your grammar knowledge from one language to the other.
In which order should you learn the languages ? Actually the order doesn’t really matter. The logical order would be to start with Chinese, focus on learning the Hanzi, learn the vocabulary, and then start Korean and/or Japanese with a head start. Characters are a great mnemonic help to learn new vocabulary. Personally I started Korean before Chinese and I had a lot of difficulty remembering the vocabulary. When I came back to Korean after learning Mandarin it all became much easier. If you start with Korean, be sure to check the etymology of all new vocabulary and note the Hanja for all Sino-Korean words. It will help you a lot for Japanese and Chinese. What is important though is to focus on just one language at a time, for a few months to a year, while regularly reviewing the others so as not to forget them.
