A quick search on the Web yields the info that Descartes lived from 1596-1650 and was educated at a Jesuit College, as well as the U. of Poitiers. The article also quoted cogito... in Latin before translating it into English. Most of the titles of his work are also listed in Latin. Ergo I assume he wrote primarily in Latin; however, one title is listed as "Traité des passions de l'âme", so he must have written some works in French. He seems to have saved French for the passionate subject matters.