1984

Why is the ruthless totalitarian regime, described in Orwell’s novel purging Oldspeak and imposing Newspeak? What assumptions are being made about the relationship between language and thought?

They are imposing Newspeak because the aim of Newspeak is to narrow the whole range of thought. They are cutting down words and cutting down the language in Newspeak. Instead of having so many words for 'good' and 'bad', in Newspeak, they will be covered in just 6 words. They want to  control people's thoughts and by imposing Newspeak, this will be easier to achieve as people's thoughts will be more clear and definitive. 

They are assuming that by narrowing the language, this will also narrow people's thoughts. They think that by making the language smaller, every concept in people's minds will be expressed by just one word. This implies that language is directly related to thought and by narrowing someone's language, it will also narrow their thoughts.