Not something I’ve done much reading on, so I’m going to quote my friend here:
“The 16th century French accent was very close to current Canadian French. It’s reinforced by the way Mantel has her saying “Cromwell” with a French accent. “Cremuel” is Cromwell with a French Canadian accent.” – @towyns
There’s also a thread on this at AB Files.
Her potential birthdate could influence how strong her French accent was, also. If she was around 12 when she left to serve Margaret of Austria, her English accent was probably very strong and firmly-set (meaning, even if she learned and became fluent in French, she would still speak English as she had, without a French accent). If she was born in 1507, her French accent would be strong because she spent earlier formative years there, and had only been fluent in her own native tongue for a few years.