Used intl ICU for message translation

docs/guide/i18n.md

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+Internationalization
+====================
+
+Internationalization (I18N) refers to the process of designing a software application so that it can be adapted to
+various languages and regions without engineering changes. For Web applications, this is of particular importance
+because the potential users may be worldwide.
+
+When developing an application it's assumed that we're relying on
+[PHP internationalization extension](http://www.php.net/manual/en/intro.intl.php). While extension covers a lot of aspects
+Yii adds a bit more:
+
+- It handles message translation.
+
+
+Locale and Language
+-------------------
+
+Translation
+-----------
+
+/*
+
+numeric arg \{\s*\d+\s*\}
+named arg \{\s*(\w|(\w|\d){2,})\s*\}
+
+named placeholder can be unicode!!!
+
+
+argName [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
+
+message = messageText (argument messageText)*
+ argument = noneArg | simpleArg | complexArg
+ complexArg = choiceArg | pluralArg | selectArg | selectordinalArg
+ noneArg = '{' argNameOrNumber '}'
+ simpleArg = '{' argNameOrNumber ',' argType [',' argStyle] '}'
+ choiceArg = '{' argNameOrNumber ',' "choice" ',' choiceStyle '}'
+ pluralArg = '{' argNameOrNumber ',' "plural" ',' pluralStyle '}'
+ selectArg = '{' argNameOrNumber ',' "select" ',' selectStyle '}'
+ selectordinalArg = '{' argNameOrNumber ',' "selectordinal" ',' pluralStyle '}'
+ choiceStyle: see ChoiceFormat
+ pluralStyle: see PluralFormat
+ selectStyle: see SelectFormat
+ argNameOrNumber = argName | argNumber
+ argName = [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
+ argNumber = '0' | ('1'..'9' ('0'..'9')*)
+ argType = "number" | "date" | "time" | "spellout" | "ordinal" | "duration"
+ argStyle = "short" | "medium" | "long" | "full" | "integer" | "currency" | "percent" | argStyleText
+ */