Looks French to me,too.
If so it should be pronounced Ros-mongd.
The S should be voiced, the stress is on the second syllable and the last vowel is typically French like in "bon" and hard to explain. It is sort of a nasal O shaped in the back of your throat. Listen to
Paul McCartney trying to sing: "
Michelle, ma belle, SONT le mots …" He is doing a fairly good job.
Andy