Theoretically you can use any text intensive game for that matter. I guess the main candidates are roleplay and adventure games, although I guess the latter tend to me more close to how people usually speak, at least if the setting is somewhat contemporary.
It depends, though. What do you understand under “informal language”? There are huge differences in how people speak in everyday life depending on age, social status, level of education as well as the region they life. And of course it depends on the matter talked about, too.
Maybe another idea would be to enter German forums?! The language level used there is usually a bit below what you encounter in everyday speech, but it could be useful for picking up slang.