In Heart of Darkness, what linguistic features define Marlow's narrative style?

The linguistic features of Marlow's narrative are actually identified by the anonymous narrator himself quite close to the beginning of the story, when he introduces Marlow and tells the reader something very important about the way in which Marlow tells stories. Whereas other seamen tell stories very simply and in a direct fashion, Marlow takes a very different approach, as the following quote demonstrates:

The yarns of seamen have a direct simplicity, the whole meaning of which lies within the shell of a cracked nut. But Marlow was not typical... and to him the meaning of an episode was not inside like  kernel but outside, enveloping the tale which brought it out only as a glow brings out a haze, in the likeness of one of these misty halos that sometimes are made visible by the spectral illumination of sunshine.

Marlow's narrative style is therefore defined not by a focus on the meaning of the story itself but the entire story and the way it is told. This is highly significant in the way that the narrative he relates has no simple and direct "meaning" or message, but is rather a complex, mutli-layered narrative with many possible meanings and interpretations. Marlow's story in this text can never be described as having a "direct simplicity," and the way in which Marlow only gradually reveals the motives and characters of some actions supports this. For example, he describes the way that his journey to Kurtz is halted inexplicably because he is not able to get the materials he needs to repair the boat, but only later does he reveal that the manager deliberately blocked the shipment of rivets in the hope that Kurtz would die in the interim. Marlow's narration is therefore full of ambiguities and mysteries, some of which are never resolved.

What does Marlow's language in Heart of Darkness reveal about him?

The story has three layers:  the author, the narrator, and Marlow, who had the original experience. The voice of Marlow might conflate with that of the narrator, but the author should not be considered the same as they. He adds to the symbolic meaning and crafts the overall themes, although the word choice comes from the narrator / Marlow. The sentence structure also characterizes the narrator and/or Marlow, which is evocative and rhythmic. These sentences in Part I provide an example of imagery and structure and, by means of them, voice: “They were not enemies, they were not criminals, they were nothing earthly now -- nothing but black shadows of disease and starvation, lying confusedly in the greenish gloom. Brought from all the recesses of the coast in all the legality of time contracts, lost in uncongenial surroundings, fed on unfamiliar food, they sickened, became inefficient, and were then allowed to crawl away and rest.” The “they” are the natives, and the details of “black shadows” and “greenish gloom” create mood as well as tone. The parallelism that concludes with “crawl away and rest,” where the language builds to a climax in the final words indicates a person gifted in story telling. This voice is attentive to detail and gives meaning to what he describes through the imagery he uses to describe it.

Well, first of all, he uses English. That's not a smart answer. The story's author wrote in English, but he came from a Polish (or Polish/Russian) family. English was alien to Conrad, and English is alien to Africa. English therefore reveals him as an outsider.

After that, he narrates the story in highly educated language (marking him as a member of civilization, rather than the "savagery" that defined Africa), formal language (pushing him into the abstract and conceptual, but also the judgmental), and high figurative, symbolic language. This last reveals his subconscious and gives the story its tone and deeper meaning.