Why don't they write: "This essay was translated from Chinese"? (Omitting the "the" before "Chinese".)
They don't write it that way because it changes the meaning to do so.
They don't mean that it was translated from the Chinese language ("Chinese"). They mean that it was translated from a specific original Chinese text ("the Chinese"). In the same way, Mr England's answer discusses "the Chinese original" because he trusts that the reader understands the implicit noun "text" or "thing". As he mentioned, this is usually understood as a form of ellipsis.
As his own usage showed, however, what's more precisely happening is the adjectives are being nominalized and standing in for their respective noun phrases via metonymy. This semantic shift is what causes the usual order of the words to change from "the original Chinese text" to "the Chinese original". As far as why that happens more often in academic writing, (a) academic writing is more liable to be carefully worded and should ideally prize concision where it can be shoehorned in. More to the point, (b) omission of implicit nouns is more common in Latin and other IE languages and has been declining in general use since English lost most of its inflections. Both aspects make its continued use seem more learned, which make it an attractive class marker for academics even (or especially?) when it becomes harder for the general public to follow their phrasing.