There Rw many hard water stain removal products. Check out the WCR Store. They are available as liquids, powders, and pastes, in addition to mechanical devices such as Mr. Hard Water and GlassRenu.
Locally, you can purchase items at hardware stores. Some products are more effective on certain stains than others. Use as instructed and use personal protective equipment (chemical gloves, mask/respirator, and goggles.)
Some are acid-based; some won't work due the level of damage already on the glass.
Use the Search feature -- there are lots of threads on the subject.