add ability to set a custom fallback translator

src/I18n/I18n.php

@@ -200,6 +200,18 @@ public static function config($name, callable $loader)
         static::translators()->registerLoader($name, $loader);
     }
 
+    /**
+     * Registers a custom fallback translator loader
+     *
+     * @param callable $loader A callable object that should return a Package
+     * instance to be used for assembling a new translator.
+     * @return void
+     */
+    public static function fallbackConfig(callable $loader)
+    {
+        static::translators()->setCustomFallbackLoader($loader);
+    }
+
     /**
      * Sets the default locale to use for future translator instances.
      * This also affects the `intl.default_locale` PHP setting.

src/I18n/TranslatorRegistry.php

@@ -35,6 +35,13 @@ class TranslatorRegistry extends TranslatorLocator
      */
     protected $_loaders;
 
+    /**
+     * Custom fallback loader
+     *
+     * @var callable
+     */
+    protected $_customFallbackLoader;
+
     /**
      * The name of the default formatter to use for newly created
      * translators from the fallback loader
@@ -146,6 +153,17 @@ public function registerLoader($name, callable $loader)
         $this->_loaders[$name] = $loader;
     }
 
+    /**
+     * Set a custom fallback loader function
+     *
+     * @param callable $loader A callable object that should return a Package
+     * @return void
+     */
+    public function setCustomFallbackLoader(callable $loader)
+    {
+        $this->_customFallbackLoader = $loader;
+    }
+
     /**
      * Sets the name of the default messages formatter to use for future
      * translator instances.
@@ -185,6 +203,10 @@ public function useFallback($enable = true)
      */
     protected function _fallbackLoader($name, $locale)
     {
+        if ($this->_customFallbackLoader) {
+            return ($this->_customFallbackLoader)($name, $locale);
+        }
+
         $chain = new ChainMessagesLoader([
             new MessagesFileLoader($name, $locale, 'mo'),
             new MessagesFileLoader($name, $locale, 'po')

tests/TestCase/I18n/I18nTest.php

@@ -469,7 +469,7 @@ public function testTranslatorCache()
      *
      * @return void
      */
-    public function testloaderFactory()
+    public function testLoaderFactory()
     {
         I18n::config('custom', function ($name, $locale) {
             $this->assertEquals('custom', $name);
@@ -510,6 +510,36 @@ public function testloaderFactory()
         $this->assertEquals('%d is 1 (po translated)', $translator->translate('%d = 1'));
     }
 
+    /**
+     * Tests that it is possible to register a fallback translators factory
+     *
+     * @return void
+     */
+    public function testFallbackLoaderFactory()
+    {
+        I18n::fallbackConfig(function ($name) {
+            $package = new Package('default');
+
+            if ($name == 'custom') {
+                $package->setMessages([
+                    'Cow' => 'Le Moo custom',
+                ]);
+            } else {
+                $package->setMessages([
+                    'Cow' => 'Le Moo default',
+                ]);
+            }
+
+            return $package;
+        });
+
+        $translator = I18n::translator('custom');
+        $this->assertEquals('Le Moo custom', $translator->translate('Cow'));
+
+        $translator = I18n::translator();
+        $this->assertEquals('Le Moo default', $translator->translate('Cow'));
+    }
+
     /**
      * Tests that missing translations will get fallbacked to the default translator
      *