Look for a file called "config.lua" in the game's directory and change the following:

localisation =
{
country = "GB",
language = "en"
},

to

localisation =
{
country = "GE",
language = "de"
},

If you don't see the config.lua file in your game's directory, you must first start the game, play a bit, and quit the game. The config.lua file will show up afterwards :)