The so-called Common Language (Gongyan, 共言) is the language of international diplomacy and trade, and, in its refined literary forms, the common language of global scholarship and culture. Basic facility with Common is considered essential to an education through the four Elemental Nations. Even in remote villages, there is likely to be someone who can speak this language.
The Common Language is derivative mostly of the language of the Central Continent of the Earth Kingdom, where it is somewhat mutually intelligible with many of the local dialects. It remains somewhat intelligible from the Abka line to the Eastern region. Related languages are spoken in the Southern Continent, but beyond these regions, Common is of different linguistic stock from the local language. After many centuries of contact, though, most other languages have extensive loanword vocabularies from Common, particularly vocabulary relating to literate culture, trade, accounting and administration.
Many people who are not really educated still speak Common, if they have anything to do with foreign trade or have to deal with the government, as, except in the Fire Nation, Common is the language of bureaucratic administration as well as diplomacy. (The language is often stereotyped phrases hacked together, and influenced by local language grammatical patterns that would make a classical scholar cringe, of course.) Part of its outsized influence is that the language was adopted from the early Earth Kingdom by other polities along with administrative systems and literature. (Common is also not necessarily the court language of polities that use it for written administrative correspondence; they may even read it aloud using local words instead of the proper Common pronunciation.)
Proficiency in reading and writing Common is a critical part of the comprehensive civil and military service examinations in the Earth Kingdom, along with knowledge of the Classics and mathematical literacy. It is also the language of instruction at Ba Sing Se University, rather than the local Ba Sing Se dialect or courtly language.
The Common Language is written with phono-ideographic characters that combine symbols conveying meaning with hints about pronunciation (often reflecting the linguistic-phonetic history of the character rather than its modern pronunciation.) It is an analytic language, and its phonology does not feature consonant clusters. It is a tonal language.
By Translation Convention, Common is represented by (mostly-)classical Chinese in the game (as in official Avatar media.)