feature #32106 [FrameworkBundle] Use default_locale as default value for translator.fallbacks (dunglas)

This PR was merged into the 4.4 branch.

Discussion
----------

[FrameworkBundle] Use default_locale as default value for translator.fallbacks

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Simplify translator's config:

Before:

```yaml
    default_locale: fr
    translator:
        default_path: '%kernel.project_dir%/translations'
        fallbacks:
            - fr
```

After:

```yaml
    default_locale: fr
    translator:
        default_path: '%kernel.project_dir%/translations'
```

Commits
-------

e0ef35973d [FrameworkBundle] Use default_locale as default value for translator.fallbacks

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php

@@ -749,9 +749,10 @@ class Configuration implements ConfigurationInterface
                     ->fixXmlConfig('path')
                     ->children()
                         ->arrayNode('fallbacks')
+                            ->info('Defaults to the value of "default_locale".')
                             ->beforeNormalization()->ifString()->then(function ($v) { return [$v]; })->end()
                             ->prototype('scalar')->end()
-                            ->defaultValue(['en'])
+                            ->defaultValue([])
                         ->end()
                         ->booleanNode('logging')->defaultValue(false)->end()
                         ->scalarNode('formatter')->defaultValue('translator.formatter.default')->end()

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -301,7 +301,7 @@ class FrameworkExtension extends Extension
         $this->registerEsiConfiguration($config['esi'], $container, $loader);
         $this->registerSsiConfiguration($config['ssi'], $container, $loader);
         $this->registerFragmentsConfiguration($config['fragments'], $container, $loader);
-        $this->registerTranslatorConfiguration($config['translator'], $container, $loader);
+        $this->registerTranslatorConfiguration($config['translator'], $container, $loader, $config['default_locale']);
         $this->registerProfilerConfiguration($config['profiler'], $container, $loader);
         $this->registerCacheConfiguration($config['cache'], $container);
         $this->registerWorkflowConfiguration($config['workflows'], $container, $loader);
@@ -1079,7 +1079,7 @@ class FrameworkExtension extends Extension
         return new Reference('assets.empty_version_strategy');
     }
 
-    private function registerTranslatorConfiguration(array $config, ContainerBuilder $container, LoaderInterface $loader)
+    private function registerTranslatorConfiguration(array $config, ContainerBuilder $container, LoaderInterface $loader, string $defaultLocale)
     {
         if (!$this->isConfigEnabled($container, $config)) {
             $container->removeDefinition('console.command.translation_debug');
@@ -1094,7 +1094,7 @@ class FrameworkExtension extends Extension
         $container->setAlias('translator', 'translator.default')->setPublic(true);
         $container->setAlias('translator.formatter', new Alias($config['formatter'], false));
         $translator = $container->findDefinition('translator.default');
-        $translator->addMethodCall('setFallbackLocales', [$config['fallbacks']]);
+        $translator->addMethodCall('setFallbackLocales', [$config['fallbacks'] ?: [$defaultLocale]]);
 
         $container->setParameter('translator.logging', $config['logging']);
         $container->setParameter('translator.default_path', $config['default_path']);

src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/ConfigurationTest.php

@@ -248,7 +248,7 @@ class ConfigurationTest extends TestCase
             ],
             'translator' => [
                 'enabled' => !class_exists(FullStack::class),
-                'fallbacks' => ['en'],
+                'fallbacks' => [],
                 'logging' => false,
                 'formatter' => 'translator.formatter.default',
                 'paths' => [],