Pecheneg language
Pecheneg is an extinct Turkic language spoken by the Pechenegs in Eastern Europe in the 7th–12th centuries. It is also possible that the language was spoken by the Cumans, per Byzantine princess Anna Komnene.
It was most likely a member of the Oghuz branch of the Turkic family, but poor documentation and the absence of any descendant languages have prevented linguists from making an accurate classification; most experts would be fairly confident in placing it among the Oghuz languages, but would refuse to classify it further.