The website or McGame and Heise are in german language only. You can change the games' language to english after you've installed it
Click on one of both game teaser images.
Click the green "Kaufen" button and add the game to your cart.
Insert the voucher code nikolaus.
The price is now €0.00
If you want both games, you have to do the above steps again for the other game.
How to change the language? Open the settings.ini file with your text editor and change these entries: "game_language = english" / "speech_sample_language = english"