add @language and @charset to "Special form variables" section

Author: Alexander Smishlajev

doc/customizing.txt

@@ -2,7 +2,7 @@
 Customising Roundup
 ===================
 
-:Version: $Revision: 1.185 $
+:Version: $Revision: 1.186 $
 
 .. This document borrows from the ZopeBook section on ZPT. The original is at:
    http://www.zope.org/Documentation/Books/ZopeBook/current/ZPT.stx
@@ -1247,13 +1247,32 @@ variables and their values. You can:
 - Remove items from a multilink property of the current item.
 - Specify that some properties are required for the edit
   operation to be successful.
+- Set up user interface locale.
 
 These operations will only take place if the form action (the
 ``@action`` variable) is "edit" or "new".
 
 In the following, <bracketed> values are variable, "@" may be
 either ":" or "@", and other text "required" is fixed.
 
+Two special form variables are used to specify user language preferences:
+
+``@language``
+  value may be locale name or ``none``. If this variable is set to
+  locale name, web interface language is changed to given value
+  (provided that appropriate translation is available), the value
+  is stored in the browser cookie and will be used for all following
+  requests.  If value is ``none`` the cookie is removed and the
+  language is changed to the tracker default, set up in the tracker
+  configuration or OS environment.
+
+``@charset``
+  value may be character set name or ``none``.  Character set name
+  is stored in the browser cookie and sets output encoding for all
+  HTML pages generated by Roundup.  If value is ``none`` the cookie
+  is removed and HTML output is reset to Roundup internal encoding
+  (UTF-8).
+
 Most properties are specified as form variables:
 
 ``<propname>``