An interesting question. The first distinction is in intention. Language in fiction can and often is intended to be pleasant in itself. Writers pay attention to sound, cadence, the literal shapes of words, etc. They might form a line for how it sounds, or for its beauty. Scientists can write beautifully, but their intention is to communicate and persuade.

A second difference is related: the relative weight of connotation and denotation. Language in fiction is always charged with connotative meaning, and is selected for emotional impact; language in science tries to be denotative.

A third difference is in focus. Fiction is about people, or about characters (animals, robots) with characteristics related to humans. Science is about the world, and can leave humans out entirely, or, if they are included, can treat them as objects rather than subjects. This means fiction uses more dialogue, more individuated descriptions, more active verbs, etc., than science.

What is the difference between the language of fiction and the language of science, Chekhov the lady with the pet dog?

One difference between the language in fiction and science is that science would more likely use a specific kind of vocabulary that might not be known to the average reader.

Another difference is that the language used in fiction is often full of 'literary devices' such as figurative language (alliteration, personification for example) and might use irony or sarcasm. An article written for science would not use figurative language or other rhetorical devices.

Finally, a difference would be that papers written for a science publication are usually written in the passive tense and in third person. A work of fiction can be either in first, second (rare) or third person and usually active.