Twilight -and subsequently Spike- using obscenities native to Ulquiorra's world makes sense as it's explicitly stated that he's teaching them such words. But where did Luna learn to use such colorful language?
Chapter 15 gives us the first serious conflict of the story, featuring a fight between Ulquiorra and a dragon. The entire fight it set to Ichigo's theme Number One, which comes off as very jarring considering the protagonist of the fight is a villain from the Bleach universe. But then you stop and realize that the scene is described from the point of view of the mane six, and Ulquiorra actually saves them from the dragon's rage. Suddenly makes a great deal more sense because they're interpreting his actions as those of a hero!