The light by a door is really meant just for lighting the passage through the entrance. To light a garden, use fixtures in the garden that are directed to throw light on plants.

Re the neighbors' blinding light, you could talk to them about your concerns and see if they're willing to re-angle the fixture or cut down on the wattage. You can also look into screening with plants and.or fencing. As a last resort, your municipality may have a code provision about light pollution or general nuisances in residential areas.

How much and how strong does the outdoor lighting have to be to be able to see through the reflection from the kitchen glass door?

It's a matter of math and logic. Light travels a path. For an object to be visible, light needs to illuminate it and then be reflected back to your eyes. Uncoated window glass will typically transmit about 96% of visible light, while reflecting 4% (and absorbing a negligible amount). Of course, that works in either direction. The outdoors will become visible when the transmitted light from outdoors is equal to (or preferably, greater) than the indoor light being reflected back to you. In your case, it appears that the outdoor light is so close to the door and angled such that most of its light is transmitted indoors, while little light illuminates the outdoor objects.

The solution is to adjust light fixtures and angles so that a flood of photons strikes the outdoor objects you want to see, while no photons enter the house directly. Additionally, indoor light needs to be adjusted so that (4% of indoor illumination) << (96% of outdoor illumination). Hope that's clear.

The problem with the light by your door is not just the reflection off the glass but the location of the light itself. Because that light is at eye level your eyes will naturally gravitate towards the brightness of that light source. When we light up homes I tell clients that in most situations what you want to see is the effect of the light but not the source of the light. As far as the light thats there now you have a few options, you can install a dimmer and lower the light level to a level more appropriate. You can make sure you have a frosted bulb instead of clear to diffuse the harshness of the bright light. Or, you can turn it off completely and rely on a different and more appropriate lighting fixture for the garden area. as for lighting up the garden area I would suggest low voltage lighting using a smaller fixture using a MR16 bulb. Try to find a location high enough that the fixture is pointing downward more that outward. Most MR16 fixtures have some sort of shroud that blocks any glare it may create from a neighbors house as long as the fixture is pointing downward enough. The ideal location would be a second story eave or a tall tree. By going with a low voltage option you can more easily run the wires in a cost effective way as you are not required to run the wiring in conduit. Now, this option will not provide as much light as your neighbor's flood light but I would argue that in most cases a lesser amount of light appears more natural and accents the area without being distracting. Using an LED MR16 allows you to choose a color of light with 2700k - 3000k creating a "warm white" color similar to halogen or incandescent and 5000k - 6000k creating a "cool white" color. You might want to try the cool white option as it creates a nice "moon lighting" type of effect when mounted in the way I've described. You can see some more examples or "down lighting" and "moon lighting" on my page.