As we mention in our section on "Genre," Howl's Moving Castle fits into the category of children's literature. Certainly, given the intended audience, Diana Wynne Jones does not get exactly Shakespearean in her language. The style of this book is both easy-to-read and entertaining—it's perfect for relaxing weekend reading or an afternoon at the beach.
Let's take a look at a passage as an example. Here Sophie has come to the King's Palace in Kingsbury to badmouth Howl because Howl desperately doesn't want to get involved in the search for the King's brother Prince Justin, since that will force him to cross paths with the Witch of the Waste. Howl believes that if Sophie poses as his old mother and talks Howl down, the King will be so impressed by the fact that not even Howl's mother trusts him with any responsibility that the King won't try to hire Howl as his official Royal Magician:
And [Sophie] found she had forgotten every word of the careful, delicate things Howl had told her to say. But she had to say something.
"He sent me to tell you he's not going to look for your brother," she said. "Your Majesty."
She stared at the King. The King stared back. It was a disaster. (13.6-8)
First off, the description of what's going on in Sophie's head is hilarious. We imagine that if we were sent to badmouth a friend of ours to the King of a fairytale country, we might also go completely blank when it came time to actually say something.
Sophie's anxiety and the simplicity of the narrator's presentation of this deeply awkward social situation both come through in the straightforward language that Jones uses. It doesn't get much more direct than: "She stared at the King. The King stared back. It was a disaster." There isn't much room for interpretation of Jones's wording.
What the narrator leaves up to us is why this social disaster is happening—what are Sophie's feelings in this scene, and what are the King's? As we discuss in our section on "Narrator Point of View," the narrator doesn't always come clean about character motivations behind their actions and reactions. However the language the narrator uses is simple enough that we can always follow the novel's descriptions of what is happening, even if we sometimes have to guess at why.