If you want to change the language of the game on Mac, please follow these steps:
1. Right-click on the game and select "Show package contents".
2. Open the following folders in succession: Contents -> Resources -> Game -> Airline Tycoon Deluxe.app -> Contents -> Resources
3. Find the "en" folder and rename it to something like "en_original".
4. Afterwards, rename the "de" or "fr" folder to "en". The game should then use the German/French files and launch in the language you selected.