The word 'koine' is correct and can be used in written English.
It is a Greek term that refers to a common or shared language or dialect. It can also be used to describe a standardized version of a language that is used as a lingua franca in a specific region. Example: "During the Hellenistic period, koine Greek spread throughout the Mediterranean as the common language of commerce and education."
(For the austere classicists of St Paul's Girls Schooll in London, a touch of koine is regarded as a "Christmas treat").In practice, few classes bond quite as tightly as the six students featured in Donna Tartt's bestselling novel "The Secret History" (in a pastiche of Euripides's "Bacchae", they commit and conceal two vicious murders).
While the koine Greek current in the eastern Mediterranean in the 1st century AD is different from the Attic, Ionian or Homeric dialects used in the greatest works of classical literature, it is also considerably easier.
The culture that resulted is known as the "Etruscan koine".
Even Modern Persian does not represent a straightforward continuation of Middle Persian but is rather a koine (a dialect or language of a small area that becomes a common or standard language of a larger area), based mainly on Middle Persian and Parthian but including elements from other languages and dialects.
It is probable that they spoke early Modern Persian and that their speech often impinged upon their writing but that they strove to write the Middle Persian of several centuries earlier as it was attested in the inscriptions of the early Sāsānian dynasty when Middle Persian was the koine.
One difficulty is that the language in a given area represents a mixture of at least two layers: an older one of the original local type, antedating the language of the Qieyun, and a younger one that is descended from the Qieyun language or a slightly younger but closely related tongue the so-called Tang koine, the standard spoken language of the Tang dynasty.
Within the alphabetical period of pre-Koine Ancient Greek (8th 4th centuries bce) there is no break between what is termed Archaic Greek (8th 6th centuries) and what is termed Classical Greek (5th 4th centuries).
Koines may be written or spoken.
It is assumed that koines also evolved in the earliest British colonies in North America, Australia, and New Zealand in response to the different metropolitan dialects that the colonists brought with them.
Since no genetically unrelated languages were involved in the contacts that produced them, the structures of koines are not as drastically divergent from those of their ancestor languages as those of creoles and pidgins.
Unlike creoles and pidgins, koines are considered to be genetically related to the language varieties from which they have evolved.
