What is the official language of Oceania?

Newspeak is the official language of Oceania in George Orwell's 1984. Information about Newspeak can be found in the appendix of the text, in the section titled "The Principles of Newspeak".
Newspeak "was the official language of Oceania and had been devised to meet the ideological needs of Ingsoc, or English Socialism" (Orwell, 309) and its official purpose
was not only to provide a medium of expression for the world-view and mental habits proper to the devotees of Ingsoc, but to make all other modes of thought impossible. It was intended that when Newspeak had been adopted once and for all and Oldspeak forgotten, a heretical thought - that is, a thought diverging from the principles of Ingsoc - should be literally unthinkable, at least so far as thought is dependent on words. Its vocabulary was so constructed as to give exact and very subtle expression to every meaning that a Party member could properly wish to express, while excluding all other meanings and also the possibility of arriving at them by indirect methods. This was done partly by the invention of new words, but chiefly through eliminating undesirable words (Orwell, 309-310).
In essence, this means that the Party devised a language whose entire purpose is to simplify language to the point where undesirable words do not exist in the language. If undesirable words do not exist in the language, then the Party believes that it will be impossible for Party members to think undesirable thoughts (or at least impossible for them to be communicated). Newspeak is therefore invaluable as a tool for controlling the thoughts, actions, and lives of Party members.
For example, it would not serve the Party for words like "bad", "unpleasant", "dissatisfied" or "terrible" to exist. If these words existed, then Party members might use these words to describe their feelings about their lives, and this might lead to dangerous thoughts of rebellion. Instead, the Party only recognizes words that are beneficial to them in Newspeak, and even then in an overly simplified form. In this example, the Party only recognizes the word "good" and other forms of this word. Instead of "bad" or "terrible" a Party member speaking Newspeak would only be able to say that something was "ungood" or "doubleungood". Since "good" has a better connotation than "bad" and because "ungood" is a less specific word than something like "unfair" or "terrible" it is difficult for Party members to communicate any unhappiness with Oceania. This limiting of thought, communication, and language makes Oceania easier to control.

Newspeak is the language of Oceania in George Orwell's dystopian masterpiece 1984. The control of language by the Party is an essential part of their totalitarian government. The central idea is that if the Party can simplify language, it can simplify people's thoughts. Eventually, as Newspeak abolishes all words associated with rebellion, the government hopes that citizens will no longer even be able to conceptualize rebellion. Control of language is the ultimate method of mind control.
Newspeak "dumbs down" the English language by taking out word variety and removing connotations. Instead of saying "bad" is the opposite of "good," Newspeak requires "ungood." This makes every idea seem positive instead of negative, which is another form of brainwashing. People will think they are happier and more satisfied with life under the regime. Winston Smith's colleague Syme is working on the Newspeak dictionary, which is why we hear about the language and its formation and evolution. In chapter 5, Syme tells Winston,

Don't you see that the whole aim of Newspeak is to narrow the range of thought? In the end we shall make thoughtcrime literally impossible, because there will be no words in which to express it. Every concept that can ever be needed will be expressed by exactly one word, with its meaning rigidly defined and all its subsidiary meanings rubbed out and forgotten.

Syme is very direct when he tells Winston that Newspeak intends "to narrow the range of thought." The simplification of language is meant to simplify people's minds and make them more malleable for Big Brother and the Party.