I saw this film at the weekend. I thought I heard that Juliette was in prison in Manchester, but my GF didn't think this. Can anyone else say where the prison was? If she was in prison in Britain this opens up a whole load of other inconsistencies, but they can wait for now.
I think she names a French city when Michel asks her where she used to live and I thought that was where the prison was because even though she gave elusive answers she did not seemed willing to lie.
She met her sister at the airport so the prison could have been anywhere. Maybe she wanted to meet for the first time in 15 years at a venue where long lost relatives meeting each other is an everyday occurence and doesnt attract a second glance.
I think she said Carcassonne (a picturesque old city in the south of France) when asked by the professor who was drawn to her. He replied that it was a nice place -- unintended irony on the professor's part, but certainly intended on the writer's part. Further ironies: the verbal similarity between Carcassonne and incarcerated, and the fact that it is a walled/fortified town.
It was definitely France because she said that she brought Pierre to the greenhouse/glasshouse (I can't remember which one!) before he died. And she lived in Carcasonne...