German is a language spoken by humans, which is commonly used in Germany.
Empath: The Luckiest Smurf stories
In the EMPATH: The Luckiest Smurf story series, Schlumpf is a language spoken by Smurfs that is identical to German.
Hero: The Guardian Smurf stories
Edward Smurftofen of the HERO: The Guardian Smurf story series occasionally speaks in German, or rather his Imaginarium-created likeness does in the game Survival.