Because The Flies is a play, the only language we can look at is the dialogue of the characters. Sartre writes his play in the tradition of the Greek tragedies that came before it. The language is intense, measured, carefully chosen and powerfully delivered. Take a look:
That was the last time, the last, I saw my youth. Suddenly, out of the blue, freedom crashed down on me and swept me off my feet. Nature sprang back, my youth went with the wind, and I knew myself alone, utterly alone in the midst of this well-meaning little universe of yours. I was like a man who lost his shadow. (3.1.130)
This isn't just a dramatic moment – this sort of speech characterizes all of The Flies. The intensity of language is well suited to the intensity of content (Sartre's existentialism wasn't exactly light cocktail chatter), and the gravity of social context (the German occupation of Paris during World War II).