I think it’s a really pretty dress and silk cotton is gorgeous but better for English weather than S of France summer. You really want to be as cool as possible there. Aspiga is a good shout, I have a couple of unlined cotton voile dresses from there and they are a godsend in hot weather.
Unless the wedding is in the evening - Mediterranean weddings often are? But then you might want something more eveningy.
You’re shopping just a bit too early for the most summery dresses to have hit the shops- give it a month and try again!