Four females and a male in a Nissan Micra died in the crash between junction 15 for Stoke-on-Trent and 14 for Stafford.
The lorry driver was arrested on suspicion of causing death by dangerous driving but has been released under investigation.
The Romanian Embassy has been informed.
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A male passenger in the front seat is in a serious but stable condition at Royal Stoke University Hospital.
The stretch of motorway was shut after the crash at about 04:30 BST on Wednesday, before reopening around lunchtime.
The driver of the Mercedes articulated lorry, a 60-year-old man from the West Midlands, was unhurt.
Police are appealing for any witnesses to the crash, or anyone who saw the vehicles before the accident, to come forward.
Specialist officers are working with the families of those who died, a spokeswoman for Staffordshire Police said.
Five people killed in a collision involving a lorry and a car on the M6 in Staffordshire were Romanian nationals, police have said.