Can I use the ground wire as a neutral for an outlet?
This is a wiring hack job.  Someone had a problem with the proper neutral wire, but they were aware that the safety grounding system has an equipotential bond with the neutral in the panel, so they are returning current on that instead.  Very dangerous; any problem in the grounding electrode system could result in all the safety grounds in your house being energized with the return current from that bulb!    Of course this arrangement would trip a whole-house RCD or GFCI.  

Alternately, the neutral wire went dead on them, so they switched both ends, so the yellow/green wire is now the neutral wire, and there's no safety ground at all.  While less unsafe, this is still bad, since Europe is really into making the safety ground wire smaller than the conductors.