Schtroumpf is a language that appears in two fanfiction story series.
Empath: The Luckiest Smurf stories
Schtroumpf is one of the "lost" languages of the Smurfs in the EMPATH: The Luckiest Smurf story series. It is equivalent to the human language of French, following the same language rules of grammar. At the present time of the story series, Schtroumpf is hardly used by any Smurf except for Painter, Sculptor, and Scribbler (who are descendants of the Schtroumpf clan) as well as Brainy, Papa Smurf, and Empath, and is mostly used for recording Smurf names in official registers. Painter in particular prefers to speak in Schtroumpf more than he does in Smurf, as he considers it the true language of the Smurfs. Vino, though also a descendant of the Schtroumpf clan, barely even knows the language. The band Smurfs Without Hats have parts in their songs that are sung in Schtroumpf.
While Papa Smurf may be referred to as Grand Schtroumpf in the Schtroumpf language, other Smurf fathers are called Père Schtroumpf while Smurf mothers are called Mère Schtroumpfette. Only Empath would refer to Papa Smurf as Père Schtroumpf due to biological relations, though because he identifies himself as being among his fellow Smurf brothers and sisters, he would still use Grand Schtroumpf.
Clockwork Smurfs stories
Schtroumpf also appears in the Clockwork Smurfs stories, used in a similar fashion as in the EMPATH: The Luckiest Smurf story series, but mostly for alternate translations of the characters' actual names since they are not actually used in-universe.
NOTE: Schtroumpf is the actual original French name for Smurf.