In one sense, language is the key to everything... because most of our communication is linguistic (not to slight the other modalities, including the visual and subliminal) and because unintelligibility locks one out of the possibility of understanding and being understood. But "a language" is inevitably a complex creature, indistinct at its edges and constantly morphing as its users invent new ways to express themselves. The many Englishes (there's a journal called World Englishes --and see History of the English Language and Atlas of North American English preliminary chapters) exemplify the problem.
There's a temptation to identify language with ethnicity, but the mapping is only occasionally 1:1 --far more often than one might like there's ambiguity in the relationship. And many people use more than one language, according to context. And then there's the question of where a dialect turns into a separate language (the traditional criterion is "intelligibility", but it's rarely so simple as yes/no).
Language standardization is a perennialy fascinating topic. Benjamin Ao's History and Prospect of Chinese Romanization traces one branch of the subject, but another to know more about is that of the Qin emperor who unified China (Qin Shi Huangdi) --who is said to have standardized the written form of the language by burning lots of books...
213 BC: Li Si, advisor to Shi Huang Di, suggests burning of all books except for technical manuals and handbooks. Confucinism is suppres sed, and much of the historical record to this time is destroyed. (www.teatime.com/tea/world/chindyn.html) --and see this for glorious detail. And The Burning of Books
The Summer Institute of Linguistics Ethnologue offers a listing of languages of China ... and ditto for Japan, North Korea , and South Korea.
Ethnologue identifies 13 varieties of spoken Chinese: Gan, Hakka, Huizhou, Jinyu, Mandarin (China's "official language", taught in schools), Min Bei, Min Dong, Min Nan, Min Zhong, Pu-Xian, Wu, Xiang, and Yue. Each also has local dialects...
Korean Profile from UCLA's Language Materials Project. See also Cantonese and Mandarin (no page for Japanese, unfortunately).
The Languages of the World by Computers and the Internet: Japanese
Welcome to the world of Chinese language from Humboldt State
(this needs a section on orthographies...)
What are writing systems really good for? Who uses them, and for what? These are non-trivial questions, and their East Asian answers are tied up with the growth of the State, with literary establishments and scholarship, and with stratification (high proportions of literate population are a recent thing...). The Examination System of imperial China, and the Examination System of contemporary Japan... both determined the fates of students.
radicals [from 1 to 17 strokes], graphemes, syllabaries, romanization, scripts, cursive, how dictionaries are organized, mechanization [typewriters, printing, computers] and other means of copying....
And written Chinese is an important element in both Japanese and Korean (though not in the written Korean of the North, which uses only the Hangul syllabary), another example of the historical webwork that binds together the disparate societies of East Asia...
Chinese writing systems from Humboldt StateThe Japanese Language (from MIT)
History and Prospect of Chinese Romanization (Benjamin Ao)
Korean Language (Stanford)