Consider the light source is the light post, outside the window of my house (Pic 1). When the rays from the light post enters through the window pane which is translucent gets scattered and forms a prominent design (Pic 2). Why does this happen?
Answer
Your window is made of patterned glass, with regular changes in thickness. This is designed to diffuse light in order to prevent people outside from seeing what's inside (or you inside from seeing out; I don't know what your neighborhood is like).
In your case, the patterns are extremely regular, so that each incoming beam of light will get spread into a wide, regular pattern, which you'd see if you shone a laser pointer through the window onto a sheet of paper (perhaps moving the laser about a bit to catch every part of the pattern).
Here, though, you have light hitting the entire window in parallel beams (from that distant streetlight), and a single recipient (your eye). This means you'll see a bright point at every location where the patterned glass bends the beam to point exactly at your eye, which produces the pattern of light that you see.
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