Enable TranslationFormBundle specific default_locale 

... so it also can be used in case the Symfony framework `default_locale` cannot be used.
Update A2lixTranslationFormExtension.php

... to not only be dependent of the kernel default_locale, but allow Bundle override.
Make `default_locale` null by default
Merge pull request #163 from Rvanlaak/master

Allow bundle to override default_locale