With the distinct northern middle English speech of Aleyn and John, we have the first example in English of an author trying to imitate a dialect not his own. There are many "northernisms" in the clerks' speech. These include the replacement of o's with a's, so that a word like "home" becomes "hame." Also there's the usage of Scandinavian loan-words like "heythen" for "however," or "lathe" for "barn." On top of that, there's the replacement of the "-lch" at the end of words with "-lk," so that a word like "whilch" (middle English for "which") becomes "whilk." It's likely that our poet attempted this dialect imitation for comedic effect, in much the same way that we might imitate an accent when telling a joke about the "Irishman who walks into a bar."
The educated clerks also talk in a much more elevated way than their enemy, Symkyn, the miller. The working-class miller probably feels alienated or excluded by the language of scholars. Just as scholarly language creates a distinct group of people who can understand and use it to participate in its conversation, the dialect of Aleyn and John marks them as somewhat alien to the southern English world in which they now live. "The Reeve's Tale" seems to draw attention to the way in which language can separate people.
The northernisms of John and Aleyn are the tale's way of emphasizing the huge social distance between the clerks and the miller.
An important theme of "The Reeve's Tale" is the way in which language can separate people into distinct groups.