This is almost certainly the case. In fact, in 1628, well before the Discourse on Method (1637) appeared, Descartes wrote an exposition of his philosophy in Latin (Regulae ad Directionem Ingenii, ‘Rules for the Direction of the Mind’); it remained unpublished during his lifetime but circulated in manuscript outside France. In the early 1630s he wrote a larger Latin work, De Mundo (‘On the World’), which he did not publish either, for political reasons. For his Discourse on Method he would have had to translate many terms and concepts from those Latin works into French.
In his discussion of Descartes, Derrida goes further. He suggests that the Latin translation of 1644 is not so much a text that refers back to an original as a return to the academic norm of the time, a restitution of philosophical discourse to its proper order, that of Latin. In addition, he argues that since at the time Latin was the written language and French the spoken language, the Latin version is also the conversion of speech into writing.
(See Jacques Derrida, ‘If There is Cause to Translate II’ in his Eyes of the University. Right to Philosophy 2, trans. Jan Plug et al., Stanford: Stanford UP 2004, p 21-24)