Quote wiki here
"French was the language of the king and his court until the end of the 14th century. During this period, marriages with French princesses reinforced the French status in the royal family.
Nevertheless, during the 13th century, intermarriages with English people became more frequent. French became progressively a second language among the upper classes. Moreover, with the Hundred Years' War and the growing spirit of English nationalism, the status of French diminished.
French was the mother tongue of the English king until Henry IV (1399–1413). He was the first to take the oath in English, and his son, Henry V (1413–1422), was the first to write in English.
By the end of the 15th century, French became the second language of a cultivated elite."
The Queen is well-versed in royal court French and speaks it extremely well as a language - not sure about that son of hers, who stumbled over his Welsh and needed many a pint to sound half-intelligible in the lingo.