Fourteenth-century English was spoken (and written) in a variety of dialects. Middle English speakers recognized three distinct dialects -- Northern, Midlands, and Southern:
Also, English though they had from the beginning three manner of speech -- Southern, Northern, and Middle speech in the middle of the land, as they come from three manner of people in Germany [i.e., Angles, Saxons, and Jutes].
[Tr. from John of Trevisa, as above.]
Modern scholars distinguish five dialects (see map).
The clerks in the Reeve's Tale are from Strother, in the Northern dialect area (north of the river Humber, which divides the Northern from the East Midland dialect area). The Reeve himself is from Norfolk, in the northern East Midlands.
Chaucer is from London. on the Thames, which divides the Kentish and East Midlands dialect areas; it is a distinct area on its own.
The Parson says he is a "Southern man," from the area south of the Thames; but he speaks in the London Dialect. He claims he can not even understand the alliterative poetry common in the North -- he uses nonsense syllables to describe it:
I kan nat geeste `rum, ram, ruf,' by lettre, (Parson's Prologue, X.43).
He may have shared John of Trevisa's attitude toward Northern Speech:
All the language of the Northumbrians, and specially at York, is so sharp, piercing, rasping, and unshapely that we Southern men can hardly understand that language. I suppose this is because they are nigh to foreign men [i.e., Scots ] and aliens who speak strangely, and also because the kings of England dwell always far from that country.
The dialect of London, the commercial, intellectual, and political center of power, was becoming the prestige dialect. The idea of "the King's English" underlies Trevisa's comment on the Northern dialect, and it appears directly in Chaucer's Treatise on the Astrolabe for the first time:
God save the king, that is lord of this langage [(Astr Pro.56-57)]
By the fifteenth century, London English was firmly established as the dialect spoken by the denizens of power, a fact used for comic effect in The Second Shepherds' Play.
The literary language that Chaucer fashioned become the standard written language of elegant writers and the language of London became the written standard for all formal English. (It is, of course, more complicated than this; for an advanced discussion see: John H. Fisher, "Chancery and the Emergence of Standard Written English in the Fifteenth Century," Speculum, Vol. 52, No. 4. (Oct., 1977), pp. 870-89.)
In the late fifteenth century, the printer William Caxton, who greatly influenced what is now Standard Written English complained about the changes in the language since earlier times and its diverse dialects:
[I] took an old book and read therein, and certainly the English was so rude and broad that I could not well understand it. And also my lord Abbot of Westminster had shown to me recently certain evidences written in old English for to translate it into our English now used. And certainly it was written in such a manner that it was more like Dutch than English. I could not translate it nor bring it to be understood.
And certainly our language now used varies far from that which was used and spoken when I was born. For we Englishmen are born under the dominination of the Moon, which is never steadfast but ever wavering, waxing one season, and wanes and decreases another season
And that common English that is spoken in one shire varies from another. Insomuch that in my days happened that certain merchants were inb a ship in the Thames, for to have sailed over the sea into Zeeland, and for lack of wind they tarried at foreland and went to land for to refresh themselves. And one of them named Sheffelde, a mercer, came into a house and asked for food; and especially he asked for eggs. And the good wife answered that she could speak no French.
And the merchant was angry, for he also could speak no French, but wanted to have had eggs, and she understood him not. And then at last another said that he woulkd have "eyren." Then the good wife understood him well.
Lo, what should a man in these days now write, "eggs" or "eyren"?
[Tr. from the preface to Enydos Caxton's Eneydos, 1490. Englisht from the French Liure des Eneydes, 1483. Ed. by the late W. [read M.] T. Culley ... and F.J. Furnivall, London, a EETS, 1890 [Widener: 11473.57].
Caxton solved the problem by using London English and thus set the standard that other printers would follow.
His puzzlement over the changes English had undergone in his lifetime will stir the sympathy of students first encountering Chaucer's language. But the problem is not all that difficult. The fifteenth century was the time of The Great Vowel Shift, which accounts for the greatest difference between Modern English and Chaucer's English, the pronunciation of the "long vowels.