[php] Add extern for `mb_language()` (#9856)

* [php] Add extern for `mb_language()`

* Using overload because the return type depends on the set of arguments

std/php/Global.hx

@@ -1093,6 +1093,12 @@ extern class Global {
 	**/
 	static function mb_check_encoding(str:String = null, ?encoding:String):Bool;
 
+	/**
+		@see http://php.net/manual/en/function.mb-language.php
+	**/
+	@:overload(function(language:String):Bool {})
+	static function mb_language():String;
+
 	/**
 		@see http://php.net/manual/en/function.mb-scrub.php
 	**/