Do not start a war use-it/vs/it-is-forbidden.
First of all, a scanner is not a radio transmitting device, it is just a radio receiver that has the ability to search some (pre-programmed or not) frequencies in a range other than just FM's 88-to-108 MHz.
Yet it is legally forbidden to use any kind of electromagnetic devices in flight, and very specially during T/O and landing, if the wheather is clear and you politely ask the crew, you should have no problem in using it, at least while cruising. At least this is how it works in Europe: a F/A told me to shut off my video camera, which I did, but I told her that we both knew that landing with reduced visibility is different from cruising with clear wheather, so she could maybe please ask the captain. Less than one minute later she was back allowing me to use the camera. That's all. The problem will be if you find a narrow-minded F/A. Pilots will NEVER tell you not to turn on a small radio.
The problem you'll find is the electromagnetic effect know as "the Faraday's cage" (don't know if this is the apropriate translation into english).
The airplane's fuselage is a closed metallic structure. While you are inside it you are, in some way, electro-magnetically isolated from outside, no matter there are lots of plastic windows. So very possible your scanner will have no coverage or very poor.
If any, try to catch a ground one somewhere in the 121s. Barcelona has 121.7 and 121.8 MHz. Athens also has 121.7
Tower is usually around 118s (Barcelona 118.1)
Once you have caught one, the last transmission of every sector usually is "call approach on 124.7", or "call radar on 124.45", or "for higher level call Marseille in 123.325. You're released by Barcelona. Good flight". This way, you'll always be able to switch to every next freq.
Hope I helped.