Use ICU locale inheritance rules

#22 added Spanish translations, but to use `es_419` we need to manually load it and set it to appear as the various child locales (`es_AR`, `es_BO` etc).
Maybe add an additional check if `translator.default` is an alias? And explicitly checking for `false` seems to be unnecessary to me since `hasDefintion()` and `hasAlias()` only return `boolean` values.

```php
if (!$container->hasDefinition('translator.default') && !$container->hasAlias('translator.default')) {
    // ...
}
```