How is it possible that the crowd, while listening to the orator during hate week, dosent seem to notice that their enemy has suddenly changed form Eurasia to eastasia?

The whole point of the Party is that the people will be so trained to accept whatever the Party wants them to believe. However, by the end of the novel, the Party almost begins to believe its own hype. They have to create an external reality very different from what reality really is. There is no truth other than the truth the party delivers. It does not matter which state is fighting which state, the truth is what the party says it is. People have been so indoctrinated through fear and carful brainwashing, they do not question anything the party claims.