Added link to ICU Locale docs in order to clarify what are Yii::$app->language values

docs/guide/i18n.md

@@ -36,6 +36,9 @@ Later you can easily change it in runtime:
 \Yii::$app->language = 'zh_CN';
 ```
 
+> **Note**: please refer to [ICU documentation](http://userguide.icu-project.org/locale) in order to find out language
+> and country codes for your case.
+
 Basic message translation
 -------------------------