Breton, Brezhoneg is a Celtic language close to Welsh or Cornish.
Breton is traditionally spoken in the western most part of Brittany “Breizh Izhel” but today many people in Rennes and Nantes - as well as Paris - know or learn Breton.
This is why it has now a unified spelling (Peurunvan) which allows for different pronunciations and therefore respects as far as possible the different dialects.
The lessons in this method are in peurunvan spelling. Some texts will give information upon dialect pronunciations (with the adapted spelling for this context).