Craft stores like Michael's have products for frosting glass. I know that they have a kind that can be painted on and that might be easier to use for a lot of windows than the spray that MH suggests.
Another thought is that there are small rods that can be attached with screws to French doors. Then you would run sheer fabric between a top and bottom rod to let light in but limit visibility. I have also seen these gathered with big fabric or ribbon bows half way up the length.