The 49-year-old, who taught French in Cairo, died after a beating in a cell on September 13th, having been detained in a police station because he did not have a valid visa.
According to the prosecution's case, six inmates in his cell had beaten him to death. They were convicted on Sunday of “assault leading to death”.
A defendants' lawyer had called into question the prosecution's case, arguing the autopsy showed he had been beaten for more than six hours with a rod and electric cables, suggesting police involvement.
Lang's mother and sister had also cast doubts on the official account, and filed complaints against police officials over the failure to rescue him.
He had been detained during a tense time in Cairo, when police and the military had been out in force to quash protests by Islamist supporters of deposed president Mohamed Morsi.
The verdict on Sunday comes as Egypt fends off accusations of police involvement in the death of Italian student Giulio Regeni, whose badly mutilated body as found after he disappeared in Cairo in January.
Egypt has denied police were involved in Regeni's death.