Create custom language not based on english I would like to add Canadian French as language, since there already is a French version of the site it would be very handy to just start from there.

When you create a new custom language it always copies the english language into the new one, you cannot select the language it should copy from. I also tried to set the default language to french and then create the new language but that doesn't change anything.

Anyone an idea how to do something like this? Or an easy way to switch the english for french after the language is copied?
Be sure to create the new custom language:

admin/config/regional/language
(use fr-CA language code)

Get the French po files from here:

https://localize.drupal.org/translate/languages/fr

Then import (choose Canadian French):

admin/config/regional/translate/import