When looking at the Far East, the written languages between the nations seem, at least at first glance, to be quite similar and for the longest time were popularly considered to be interchangeable. While nowadays, the distinctions are marginally clearer and calling all those vastly different cultures the same is more than a little ignorant, it is not unreasonable for those not too exposed to those languages to be confused. The similarity of the writing systems has led to them being collectively known as CJK, referring to China, Japan and Korea, wherein this similarity is most evident, though occasionally Vietnam is also included.
As with most similarities between these cultures, the reason is simply China and its extremely long history. As a massive empire and the single most influential power in the Far East, China has had a major part in developing most cultures within its reach in a variety of ways. However, despite this common point for all the languages within the CJKV, why is it that speaking any one of these languages, does not mean you can speak or read any of the others? This is due to the nature of what exactly was taken from China and how exactly the JKV ended up developing that over the years.
Before China influenced them, Japan had a spoken language but no writing system, so they ended up using the Chinese writing system instead. The language they developed on their own had no resemblance to Chinese whatsoever but attributed their words to the Chinese characters, called hànzì in China, while keeping their own grammar.
This resulted in a whole new original way of reading the same characters that China developed, adopted under the name Kanji, completely unique to Japanese. Overtime, Japan went on to create two more scripts, Hiragana and Katakana, unique to them, to help supplement the differences. This meant that a Chinese person could look at a Japanese sentence and glean some very rudimentary understanding of the meaning, but could not speak it in Japanese, and vice versa.
Similarly, Vietnam and Korea also adopted the Chinese writing system for their own languages, but their relationship with it is a bit shakier. Vietnam, originally a part of China, continued to use Classic Chinese as a writing system even after its independence alongside ‘chữ Nôm’, a Chinese based writing script with locally invented characters for Vietnamese words not found in the Chinese language. Use of ‘chữ Nôm’ declined to near extinction after French colonial rule of Vietnam, during which time the modern Vietnamese alphabet was created, based on the Latin alphabet.
Korea adopted the Chinese writing system under the name Hanja and continued its use much in the same way as Japan and Vietnam. However, Hanja was commonly known only among the noble class while the common people were largely illiterate. This changed in the 15th Century, when King Sejong the Great personally created Hangul, the modern Korean alphabet, in order to supplement or replace Hanja, which he felt was not enough to write Korean.
Due to the relative simplicity of Hangul relative to Hanja, it quickly took its place as the official writing system of Korea. Even now that Korea has split, though North Korea has stopped its use in an official capacity, the South still uses Hanja, though rarely.
So, ultimately, the similarities between the CJKV are quite evident and it is not wrong to mistake them with one another. However, to ignore the differences is to ignore the centuries of history and development that each of these nations went through in developing their own language and culture. By recognizing their uniqueness, we can give them the respect that is due.