Actually you answered my question perfectly.
I had the idea of compiling a list of all airports present in the local copy of Aerofly FS2 so users could select them for the Wettergerät app. Presently Wettergerät's users have to enter the ICAO airport code manually if it is not present in their flight plan.
I also had the idea that the Wettergerät optionally sets the starting position of the plane to the airport the user fetches the weather data for. But like you said: one needs to know much more about the airport in order to solve this.
Thank you for your insights.