Language can be used to manipulate, as well as an instrument of control. The pigs in Animal Farm use rhetoric to twist and distort what is really occurring on the farm; their language is intended to justify their actions and to pacify the other animals. An example of this would be the animal's respect for Major. They respected his vision of what could be, but after his death the pigs manipulate his words. As a result, they don't meet with any resistance from the others because they initially believed in the rebellion. To oppose the pigs would mean they see their actions were wrong, that the rebellion wasn't what they expected it to be.
Another example is the Seven Commandments and their constant revision. What began as equality, is redefined in terms of some creatures being more "equal" than others. Orwell uses this language to compel the reader to "think" and follow how manipulation actually works, and to realize its effects are often unconscious responses.