French, 27, who has dual British and Norwegian nationality, and his friend Tjostolv Moland, 28, a former Norwegian soldier, were convicted of killing a taxi driver and spying.
Both men deny the murder of Abedi Kasongo, 47, a taxi driver who was shot in the head outside Kisangani in May. They claim unknown gunmen ambushed them as they were being transported in the car through a dense rainforest.
A military appeal court also ordered both men to pay £300 million in compensation before their execution.
Human rights group Reprieve claims the two men have suffered a series of miscarriages of justice since their arrest, including beatings and mock executions.
It said that during the reading of yesterday's judgment, the judge complained the interpreter was too slow and asked him to stop translating.
It also sayd Mr Moland has been suffering from cerebral malaria and hallucinations and believes he has spiritual contact with a pygmy.
Clive Stafford Smith, Reprieve director, said: "Today, in the farce that substitutes for military justice in the DRC, the judge wanted to dispense with translators.
"Apparently Joshua French did not have to understand the process that led to his death sentence.
"The 'legal process' has so far included torturing Joshua, parading him around town in front of lynch mobs, and forcing him to sign statements under threat of death."
French was born in Norway to a British father and Norwegian mother, but grew up in Margate, Kent. He moved back to Norway when his parents divorced, but returned to Britain aged 20 and served in the British Army. He also served in the Norwegian Army where he met Moland.
They both left the Norwegian Army in 2007 and worked as security guards in various places, including the Gulf of Aden where they guarded against pirates.