Add fallback to Cajun French

This change adds French (français, fr) as a fallback language to
Cajun French (français cadien, frc). If Louisiana Creole French
(kréyol la lwizyàn, lou) is added to translatewiki, it may be added
as a fallback.

Change-Id: I13fab20d930e86daf1da36f4fa382465123e74a0