diff --git a/docs/guide/i18n.md b/docs/guide/i18n.md
index d126dee..d2ecb5e 100644
--- a/docs/guide/i18n.md
+++ b/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
 -------------------------