Allow bundle to override default_locale

... in case `kernel.default_locale` cannot be used.

I was unable to fork the documentation, seems to be broken?
http://symfony.com/doc/current/components/config/definition.html#default-and-required-values isRequired doesn't manage argument.
My configuration and clear your cache:

````
david@david-PB-Laptop:/srv/www/Demo$ php ../composer.phar run-script post-update-cmd
Updating the "app/config/parameters.yml" file


                                                                                        
  [Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]         
  The child node "default_locale" at path "a2lix_translation_form" must be configured.  
                                                                                        


Script Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache handling the post-update-cmd event terminated with an exception


                                                                             
  [RuntimeException]                                                         
  An error occurred when executing the "'cache:clear --no-warmup'" command.  
                                                                             
````