English is of course the pre-requisite.
If you could learn another language to that would be advantageous in aviation what would it be?
The language of the country you're working in. If that happens to be English, then the second most popular (eg USA = Spanish).
What would be the most advantageous language to know to work within Europe?
If all the European languages were evenly split, I'd go for French - comprenez-vous ? At least you'll be able to read an original Airbus/Eurocopter manual !
