Breton, Brezhoneg is a Celtic language close to Welsh and Cornish.
Breton is traditionally spoken in the westernmost part of Brittany (Breizh Izel), but today many people in Roazhon/Rennes and Naoned/Nantes - as well as in 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 dialectal pronunciations, with the adapted spelling for that context.