Bag-snatching, theft, pick-pocketing and petty crime is no more a problem in Marrakesh than any other city or popular tourist destination.
Most of the concerns about women's safety in Morocco relate to the level of harassment they receive. It rarely gets seriously physical (a little inappropriate touching at worst), but leering, hissing and lewd comments are constant and can wear you down.
The best advice is to not engage the men in anyway, don't answer their greetings or react to their comments, simply ignore them and go about your business. However, if you were to respond I think you're understanding of French will be very useful (practice a few put-dons before you go).
I suspect you're not the type to be wearing skimpy shorts and a halter-neck t-shirt, and will be modestly dressed which will go a long way to reducing the harassment. Try wearing a headscarf "Berber-style", with little more than the eyes showing, to further blend in.
Don't walk up dark, deserted alleyways, stay aware to your surroundings, don't flash valuables and everything should be fine.
I'm told the main square of the old medina, the Djemaa El-Fna, at dusk is a sight not to be missed. Snake-charmers and lots of families enjoying the cool evening. Wish I was going with you. Phil over 6 years ago
When I was in Marrakesh in 2006 I met a 70-something lady there who was travelling alone, she was staying in my hostel and had only good things to say about her trip.
Everything Phil has said is correct. Although there is a fair amount of catcalling from younger guys to younger foreigners (like myself), there was a lot more respect for older women. It's great that you speak French, many of the older gentlemen will happily converse with you or help you out if need be. The majority of men are very nice if you try to show a little interest in their culture. Likewise, women are very friendly too but a little more shy than the men.
The Djemaa El-Fna is wonderful, particularly if you are an adventurous foodie.
Have no fear! Enjoy your trip!
Anna about 6 years ago