Simple, 16th-Century English, Highly Descriptive
So, did you believe they were reading a novel from the 16th century? If so, Mark Twain did a pretty good job, since he was born over 200 years after the events in this novel.
Twain mixes his modern, simple prose with the archaic diction of Henry VIII's day (pretty much the kind of language you see in Shakespeare's plays). So what does that look like? Well, sometimes we see plain, modern sentences like this: "When we saw him last, royalty was just beginning to have a bright side for him" (30.2). And then we jump to old-timey sentences like this: "Fathers be alike, mayhap. Mine hath not a doll's temper. He smiteth with a heavy hand, yet spareth me: he spareth me not always with his tongue, though, sooth to say. How doth thy mother use thee?" (3.31)
The first sentence is clear as day, and the second might require the use of an English-to-English dictionary just to get the meanings of the words. By mixing these styles, Mark Twain gives us both a feeling of authenticity and an ease of reading that might not have been possible otherwise. (Can you imagine if he had written this entirely in 16th-century English? Perish ye olde thought.)
Twain emphasizes the time period by being highly descriptive about the surroundings and the characters clothes. Here's how he describes Tom, for example: "He was magnificently habited in a doublet of white satin, with a front- piece of purple cloth-of-tissue, powdered with diamonds, and edged with ermine. Over this he wore a mantle of white cloth-of-gold, pounced with the triple-feather crest, lined with blue satin, set with pearls and precious stones, and fastened with a clasp of brilliants" (9.6).
Yeah, that's just a little different from what the guys were wearing in Mark Twain's day. (We think ermine and doublets were out of fashion by then.) In this case, Twain is emphasizing historical detail (in this case descriptions of clothing), but he's doing it in easy-to-understand language. That's pretty characteristic of the whole novel: just enough history to make it real, but not too much that we can't understand what's going on.