Clémentine de Rozières? In one of several interviews to commemorate his 30th birthday, Hereditary Grand Duke Guillaume acknowledged he is involved in a relationship with a foreigner. They have known each other for several years, but only became a couple six months ago. I was told that the name was first published on the Noblesse et Royautes blog without any verification.
Clémentine studied at the Institut d'Etudes politiques de Paris (2001-2006), and now works as the assistant to the general manager for Proctor & Gamble in Luxembourg.