Geoffrey Chaucer probably spoke French from his earliest age, for when he was born, the custom was still as Ranulph Higden (died 1364) described it a few years earlier:
Children in school, contrary to the usage and custom of other nations, are compelled to drop their own language and to construe their lessons and other tasks in French, and have done so since the Normans first came to England. Also, gentlemen's children are taught to speak French from the time that they are rocked in their cradles and can talk and play with a child's toy; and provincial men want to liken themselves to gentlemen, and try with great effort to speak French, so as to be more thought of.
(Polychronicon Ranulphi Higden monachi Cestrensis . . . ed. Rev. Joseph Rawson Lumby, [Liechtenstein] Krause Reprint, 1965 [Widener: Br 98.86.1].)
This unusual situation, in which the common people spoke one language, and the aristocrats another, was due to the Norman Invasion in 1066. Robert of Gloucester, a late 13th-century chronicler (fl. 1260-1300; his Chronicle ends in 1271) tells how this came about:
Much sorrow has been often in England,
As you may hear and understand,
Of many battles that have been and men have conquered this land.
First, as you have heard, the emperors of Rome.
Then the Saxons and Angles with battles strong,
And then those of Denmark that held it so long,
At last those of Normandy that be yet here Won it and hold it yet;
I will you tell in what manner:
When William the Bastard heard tell of Harolds' treachery.
How he had made him king and with such falsehood,
For that land was given to him, as Harold well knew
. . .
Thus came -- lo! -- England into Norman's hands,
And the Normans could not speak anything except their own speech,
And spoke French as they did at home, and their children did also teach,
So that high men of this land that of their blood come Hold to all that speech that they took of them;
For unless a man knows French, men think little of him.
But low men hold to English and to their own speech yet.
I suppose there be none in all the countries of the world
That do not hold to their own speech, save for England alone,
But yet it is well for a man to know both,
For the more a man knows the more he is worth.
(Tr. from The metrical chronicle of Robert of Gloucester, ed. William Aldis Wright. [Liechtenstein] Kraus Reprint, 1965 [Widener: Br 98.41].)
Chaucer was of the gentle classes and he clearly spoke French from an early age and probably first wrote poems in French, the language of the courts in which he served first as a page in the court of the Countess of Ulster and then as squire in the courts of Prince Lionel and Kings Edward III and Richard II.
The situation was changing in Chaucer's lifetime -- or rather, changes that had been operating since the thirteenth century were beginning to have an obvious effect. The aristocracy used French but most used English as well. King Edward I knew English and even enjoyed English poetry. However, French continued its cultural dominance: The court of King Edward III was French in culture and cultivated French poetry, with French poets such as Jean Froissart and Otho de Graunson, whom Chaucer knew, helping to set the tone. Furthermore the court began speaking Parisian French, an acquired skill, rather than Anglo-Norman, the variety of French used in England, to which earlier nobles had been born. By the time Chaucer wrote The Canterbury Tales the form of speech brought over by the Normans was still spoken only in the provinces, a source of gentle satire in the portrait of the Prioress:
And Frenssh she spak ful faire and fetisly,
After the scole of Stratford atte Bowe,
For Frenssh of Parys was to hire unknowe.
(General Prologue, I.124-26)
By this time, English had replaced French as the language of instruction in the elementary schools. John of Trevisa, who translated Higden's Polychronicon, quoted above, says that now, at the time of his writing (1385), the situation has greatly changed:
This manner [of instruction in French in elementary schools] was much used before the first plague  and is since somewhat changed. For John Cornwal. a master of grammar, changed the teaching in grammar school and the construing of Latin into French into English; amd Richard Pencrych learned that manner of teaching from him, and other men from Pencrych. so that now, the year of our lord one thousand three hundred four score and five, of the second King Richard after the Conquest nine, in the grammar schools of England children leave French and construe and learn in English, and they have advantage on one side and disadvantage on another. Their advantage is that they learn their grammar in less time than they were accustomed to do. The disadvantage is that now children of grammar school know no more French than their left heel, and that is harmful for them if they should pass the sea and work in strange lands, and in many other cases. Also gentlemen have now much left off teaching their children French.
(Tr. from the edition cited above.)
English was also becoming the language of government; in 1362 Parliament was opened with a speech by the Chief Justice in English (and by the Chancellor in the next two parliaments), the first time since the Conquest the native language was so used. Also in the Parliament of 1362 the Statute of Pleading was enacted. It provided that
All pleas which shall be pleaded in his [the King's] courts whatsoever, before any of his justices whatsoever . . . shall be pleaded, shewed, defended, answered, debated, and judged in the English tongue.
Though the statute also specified that the records of pleas were to be kept in Latin (and the parliamentary speeches were recorded in French), by this time English was coming to be regarded as a language suitable for aristocratic literature. In the early fourteenth century English writers aimed for audiences that knew no French. Robert Manning began his Story of England (finished 1338) with:
Lordinges that be now here,
If ye will, listen and lere
Al the story of Ingland
As Robert Mannyng written it fand
And on Inglysch has it schewed,
Not for the lerned but for the lewed
For tho that in this lande wone
That the latin ne frankys cone.
. . . [he lists his sources]
As thai haf writen and sayd,
Haf I al in my Inglish layd
In simple speche, as I couthe,
That is lightest in mannes mouthe
know no Latin or French
Robert Manning wrote specifically for the "lewed," the unlearned. His contemporary, the author of the early fourteenth-century Arthour and Merlin claims to write for even a noble audience:
French use these gentlemen,
But everone understands English;
Many a noble I have seen
That could speak no French.
By the later fourteenth century a demand for English had developed, and literary works in English were wanted not because their audience had no French but because they preferred English. John Gower wrote works in Latin, French, and English -- the latter, his Confessio Amantis, written at the request of King Richard himself.
Chaucer, as noted above, probably wrote his earliest poems in French, but none have survived (unless those poems marked with the cryptic "Ch" in the Pennsylvania MS are Chaucer's: see Wimsatt, James I. (ed), Chaucer and the poems of "Ch" in University of Pennsylvania MS French 15, Cambridge, [Cambridgeshire]: Brewer ;Totowa, N.J., USA: Rowman & Littlefield, 1982 [PR1911.W56 1982].) He was known rather as an English poet, the most respected of the time, and the respect he received is a measure of the respect English had gained as a literary medium.
Nevertheless, Chaucer remained very much aware of the problem of writing sophisticated poetry in English; it demanded a new form of the language -- a literary language, shaped largely by French and Latin models -- the high style -- and with a heavy use of borrowings from Latin and French but built upon the old popular tradition that Chaucer knew as a boy (and fondly pokes fun at in Sir Thopas) and on a keen awareness of actual speech, which forms the basis of his dramatic style.
For a bibliography of critical and scholarly works on Chaucer's language and style, click here.