You can improve it, but you'll never get rid of it completely, its a lot of effort for very little reward.
Using a razor scaper, either the 4" Unger one, or the smaller Stanley one for instance can help if the build up is really bad, but it will be almost etched into the glass, and you will not remove it all.
You can also use the finest grade of wire wool (ooo?) in conjunction with a product such as MDR, but you still have to soak and leave the glass, the acid has to be given time to work.
limescale deposite gets on many windows, its leached out of brickwork and mortar by rain.
Much easier to tell the customer there is nothing you can do about it, either that or tell them you'll have to quadruple the price you charge plus the cost of the chemicals needed, and it won't get rid of it completely even then.
Best of luck,
Ian