Docs about handling missing translations added

docs/guide/tutorial-i18n.md

@@ -417,6 +417,52 @@ Sometimes you want to correct default framework message translation for your app
 
 Now you can place your adjusted translations to `/path/to/my/message/files`.
 
+### Handling missing translations
+
+If the translation is missing at the source, Yii displays the requested message content. Such behavior very convenient
+in case your raw message is a valid verbose text. However, sometimes it is not enough.
+You may need to perform some custom processing of the situation, when requested translation is missing at the source.
+This can be achieved via 'missingTranslation' event of the [[yii\i18n\MessageSource]].
+
+For example: lets mark all missing translations with something notable, so they can be easily found at the page.
+First we need to setup event handler, this can be done via configuration:
+
+```php
+'components' => [
+    // ...
+    'i18n' => [
+        'translations' => [
+            'app*' => [
+                'class' => 'yii\i18n\PhpMessageSource',
+                'fileMap' => [
+                    'app' => 'app.php',
+                    'app/error' => 'error.php',
+                ],
+                'on missingTranslation' => ['TranslationEventHandler', 'handleMissingTranslation']
+            ],
+        ],
+    ],
+],
+```
+
+Now we need to implement own handler:
+
+```php
+use yii\i18n\MissingTranslationEvent;
+
+class TranslationEventHandler
+{
+    public static function(MissingTranslationEvent $event) {
+        $event->translatedMessage = "@MISSING: {$event->category}.{$event->message} FOR LANGUAGE {$event->language} @";
+    }
+}
+```
+
+If [[yii\i18n\MissingTranslationEvent::translatedMessage]] is set by event handler it will be displayed as translation result.
+
+> Attention: each message source handles its missing translations separately. If you are using several message sources
+  and wish them treat missing translation in the same way, you should assign corresponding event handler to each of them.
+
 Views
 -----
 
@@ -444,7 +490,7 @@ return [
 ];
 ```
 
-After cofiguring the component can be accessed as `Yii::$app->formatter`.
+After configuring the component can be accessed as `Yii::$app->formatter`.
 
 Note that in order to use i18n formatter you need to install and enable
 [intl](http://www.php.net/manual/en/intro.intl.php) PHP extension.