Like everything else in this novel, language is a mixed bag. On the one hand, Marlow sees Kurtz's eloquence as a redeeming feature—but it's also the reason he goes mad. In the end, he's "very little more than a voice" (2.27).
But we could also say that language is used as a human connection. When Marlow finds the harlequin's book, he feels relieved because he can connect to something manmade: "The simple old sailor, with his talk of chains and purchases, made me forget the jungle and the pilgrims in a delicious sensation of having come upon something unmistakably real" (2.9).
So what? Well, maybe Marlow's whole story is a way to differentiate himself from Kurtz: it's a symbolic way of retaining his own individuality among the encroaching madness brought upon by the wilderness.
Hm. Sounds good to us.