Khrysta is right, but if you misunderstood his post, here's how it works.
What he's saying is that if you breed Vivillon together, even if they have different patterns, the baby Scatterbugs will always evolve into the Vivillon pattern that is assigned to your region. Say for example I had a male Marine Vivillon bred with a female Elegant Vivillon, if I bred the two the Scatterbug produced from the egg will still evolve into a River Vivillon in my case, since that's the pattern my region gets.
The pattern is basically the same as the one you get from easily evolving a Scatterbug caught in the wild, and this changes based on what your region settings were at the time you started your save file. Example is: your region setting was New York, USA. That region gets Polar Vivillon so that means that if you were to breed, that will be the pattern your baby Scatterbug will evolve into.
As Khrysta mentioned, some countries get more than one pattern, and the one that's assigned is then broken down into smaller regions in that country.
Click here for a map of which parts of the world get which specific Vivillon forms. There's a colored name under each Vivillon pic shown which match the color highlighted parts that correspond with the pattern.
