If you ask about a second language for europe you have to define which europe you mean. In any case the biggest native speaker language group in the EU-europe is german with 93 million native speakers, second is english with around 61 million native speakers followed by french (59) and italian (58) with spanish being a distant 5th if you include catalan with 39 native speakers.
If you count in the other parts of europe, especially native russian speakers, you would find that the biggest native speaker language group in europe is russian.
Well, that doesn't really help at all since russia is pretty much a closed market for pilots, however french, german or italian should be good choices, if you want to work worldwide including south america spanish is the natural choice, arab could be interesting if you want to work in the middle east.
BTW, sorry for my poor english, it is just a secondary or tertiary language for me...