More specific description of language property of the application

docs/guide/i18n.md

@@ -36,8 +36,12 @@ 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.
+Format is `ll-CC` where `ll` is  two- or three-letter lowercase code for a language according to
+[ISO-639](http://www.loc.gov/standards/iso639-2/) and `CC` is country code according to
+[ISO-3166](http://www.iso.org/iso/en/prods-services/iso3166ma/02iso-3166-code-lists/list-en1.html).
+
+> **Note**: you can further customize details specifying language
+> [as documented in ICU project](http://userguide.icu-project.org/locale#TOC-The-Locale-Concept).
 
 Basic message translation
 -------------------------