Until Chaucer's time French and Latin were the languages of sophisticated literature (See the section on The Middle English Language in the pages on Language and Linguistics). In his lifetime English displaced French as the language of aristocratic literature. Chaucer's own works had a powerful influence on this development, so much so that later writers, such as Spenser, regarded him as the founder of the language itself, the "well of English undefiled." Browsing through the Oxford English Dictionary (which lists the first recorded uses of its entries) might convince one that Spenser was right; scores and scores of words "borrowed" from Latin or French are recorded as first appearing in Chaucer's works.
However, scholars now recognize that the language we speak today would have developed pretty much as it did if Chaucer had never written. But Chaucer did have an important influence on the language of English literature; he shaped the language of aristocratic literature for the poets who followed (and often revered) him.
He greatly expanded the language of poetry -- partly because he greatly extended its subject matter, bringing into poetic discourse philosophy, science (alchemy and astrology), trade (mainly as represented by the pilgrims), and everyday life (drawing upon everyday language of the sort that had never been used in English literature).
So far as his admirers and imitators were concerned, Chaucer's greatest contribution to the literary language was his invention of the High Style. This style is characterized by a heavy use of "borrowings" from the romance (Latin and French) languages, and in his works many romance words appear in English for the first time.