Improve and document the language-change event on MS-Windows.

 src/keyboard.c (kbd_buffer_get_event): Include the codepage and the
 language ID in the event parameters.
 src/w32term.c (w32_read_socket): Put the new keyboard codepage into
 event.code, not the obscure "character set ID".
 doc/lispref/commands.texi (Misc Events): Document the language-change event.

doc/lispref/ChangeLog

+2012-06-23  Eli Zaretskii  <eliz@gnu.org>
+
+	* commands.texi (Misc Events): Document the language-change event.
+
 2012-06-22  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Support higher-resolution time stamps (Bug#9000).

doc/lispref/commands.texi

@@ -1716,6 +1716,38 @@ To test the signal handler, you can make Emacs send a signal to itself:
 @smallexample
 (signal-process (emacs-pid) 'sigusr1)
 @end smallexample
+
+@cindex @code{language-change} event
+@item language-change
+This kind of event is generated on MS-Windows when the input language
+has changed.  This typically means that the keyboard keys will send to
+Emacs characters from a different language.  The generated event has
+this form:
+
+@smallexample
+(language-change @var{frame} @var{codepage} @var{language-id})
+@end smallexample
+
+@noindent
+Here @var{frame} is the frame which was current when the input
+language changed; @var{codepage} is the new codepage number; and
+@var{language-id} is the numerical ID of the new input language.  The
+coding-system (@pxref{Coding Systems}) that corresponds to
+@var{codepage} is @code{cp@var{codepage}} or
+@code{windows-@var{codepage}}.  To convert @var{language-id} to a
+string (e.g., to use it for various language-dependent features, such
+as @code{set-language-environment}), use the
+@code{w32-get-locale-info} function, like this:
+
+@smallexample
+;; Get the abbreviated language name, such as "ENU" for English
+(w32-get-locale-info language-id)
+;; Get the full English name of the language,
+;; such as "English (United States)"
+(w32-get-locale-info language-id 4097)
+;; Get the full localized name of the language
+(w32-get-locale-info language-id t)
+@end smallexample
 @end table
 
   If one of these events arrives in the middle of a key sequence---that

src/ChangeLog

+2012-06-23  Eli Zaretskii  <eliz@gnu.org>
+
+	* keyboard.c (kbd_buffer_get_event): Include the codepage and the
+	language ID in the event parameters.
+
+	* w32term.c (w32_read_socket): Put the new keyboard codepage into
+	event.code, not the obscure "character set ID".
+
 2012-06-23  Chong Yidong  <cyd@gnu.org>
 
 	* xmenu.c (x_menu_wait_for_event): Adapt GTK3 to new xg_select.

src/keyboard.c

@@ -3982,9 +3982,11 @@ kbd_buffer_get_event (KBOARD **kbp,
 #if defined (WINDOWSNT)
       else if (event->kind == LANGUAGE_CHANGE_EVENT)
 	{
-	  /* Make an event (language-change (FRAME CHARSET LCID)).  */
-	  obj = Fcons (event->frame_or_window, Qnil);
-	  obj = Fcons (Qlanguage_change, Fcons (obj, Qnil));
+	  /* Make an event (language-change (FRAME CODEPAGE LANGUAGE-ID)).  */
+	  obj = Fcons (Qlanguage_change,
+		       list3 (event->frame_or_window,
+			      make_number (event->code),
+			      make_number (event->modifiers)));
 	  kbd_fetch_ptr = event + 1;
 	}
 #endif

src/w32term.c

@@ -4234,8 +4234,8 @@ w32_read_socket (struct terminal *terminal, int expected,
 	  /* Generate a language change event.  */
 	  f = x_window_to_frame (dpyinfo, msg.msg.hwnd);
 
-	  /* lParam contains the input lang ID.  Use it to update our
-	     record of the keyboard codepage.  */
+	  /* lParam contains the input language ID in its low 16 bits.
+	     Use it to update our record of the keyboard codepage.  */
 	  keyboard_codepage = codepage_for_locale ((LCID)(msg.msg.lParam
 							  & 0xffff));
 
@@ -4243,7 +4243,7 @@ w32_read_socket (struct terminal *terminal, int expected,
 	    {
 	      inev.kind = LANGUAGE_CHANGE_EVENT;
 	      XSETFRAME (inev.frame_or_window, f);
-	      inev.code = msg.msg.wParam;
+	      inev.code = keyboard_codepage;
 	      inev.modifiers = msg.msg.lParam & 0xffff;
 	    }
 	  break;