Wikipedia articles ought to be written in plain English, states Wikipedia's Manual of Style.

The art of writing plain English has to be learned. Strunk and White's The Elements of Style and Gowers' The Complete Plain Words are two justly famous manuals that try to teach it. The guidance in manuals such as these is not universally applicable – Swinburne or Jeremy Taylor rewritten to comply with it would lose all interest – but it does apply unequivocally to Wikipedia articles.

An encyclopedia article is a piece of expository prose, and as such it has an objective purpose. This purpose is not to impress its readers with your learning or vocabulary, even if that is your subjective purpose for writing it. Instead, its purpose is to impart information, whether by introducing new knowledge to people who lack it or by reminding people of what they had half-forgotten. Plain words best serve this purpose.

As William Strunk put it:

Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all his sentences short, or that he avoid all detail and treat his subjects only in outline, but that every word tell.

Ernest Gowers gave many examples of writing that could be improved. This is one from the UK Government about police complaints procedures:[1]

Use all the words you need, but no more.

Use all the words you need, but no more.