Fix issue with Fallbacks raising I18n::InvalidLocale errors when enforce_available_locales is set

Trying to silence the recently added warning about `enforce_available_locales` generates errors in Rails as described in https://github.com/rails/rails/issues/13164.

I18n is not handling correctly the case where `enforce_available_locales` is set and some fallback module is not available (e.g. user provided translations for `:'de-DE'` but not for the automatic fallback `:'de'`).