I would be very careful with this problem. Before you start get a 'scratched glass' waiver from the builder or property owner.
The cement should have been removed by whoever put it there. As you are left with the job it would be better done by hand so explore that way first. Is the scaffold still up? Could the rendering guys be compelled to get the rendering off the glass?
Any bits of rendering left on the glass will scratch it with WFP. At least speak to the property owner about it first.