The main linguistic device that Mark Twain used was dialect, or allowing different groups to speak differently.
Mark Twain explains in the “Explanatory” that different characters in the book speak differently. This is known as dialect, which means writing how people talk. They are all forms of English, but different kinds of English. Huck speaks differently than the slaves, and they all speak differently than most Americans today. This is a combination of the unique Southern dialect and the time period pre-Civil War.
Jim’s slave dialect is quite distinctive.
“Say, who is you? Whar is you? Dog my cats ef I didn' hear sumf'n. Well, I know what I's gwyne to do: I's gwyne to set down here and listen tell I hears it agin.” (Ch 2)
Here, Twain takes great pains to write how people speak, or how Jim would have spoken. This gives Jim his own unique voice. Huck’s speech patterns also demonstrate who he is.
YOU DON'T KNOW about me without you have read a book by the name of The Adventures of Tom Sawyer; but that ain't no matter. That book was made by Mr. Mark Twain, and he told the truth, mainly. There was things which he stretched, but mainly he told the truth. (Ch 1)
These speech patterns reinforce that Huck is young, uneducated, and from the South. All of these dialects help set the stage, bringing the reader deeper into the story. We hear things as Huck would have heard them, and Huck speaks to us as he would have a friend. It adds authenticity to the narrative, and helps the reader become part of the story.