This question already has an answer here:
Windows XP because its the most stable system ever ?
C++ because its the most verstatile thing ever ?
Also I am not limiting this question to avionics but to
Garmin GPS/Glass cockpit
B737 Flight systems
Airbus A319 flight systems
F4E Phantom II flight systems
Bell 206 flight systems
In here, Flight systems can mean anything from basic fuel systems to autopilot systems
So which OS/Language does the aviation use ?