X11: Fix CJK IME input when locale CTYPE is "C"

Fixes #1587.
Fixes #1636.

README.md

@@ -167,6 +167,7 @@ information on what to include when reporting a bug.
  - [X11] Bugfix: Window position events were not emitted during resizing (#1613)
  - [X11] Bugfix: `glfwFocusWindow` could terminate on older WMs or without a WM
  - [X11] Bugfix: Querying a disconnected monitor could segfault (#1602)
+ - [X11] Bugfix: IME input of CJK was broken for "C" locale (#1587,#1636)
  - [Wayland] Removed support for `wl_shell` (#1443)
  - [Wayland] Bugfix: The `GLFW_HAND_CURSOR` shape used the wrong image (#1432)
  - [Wayland] Bugfix: `CLOCK_MONOTONIC` was not correctly enabled

include/GLFW/glfw3.h

@@ -1831,6 +1831,10 @@ typedef struct GLFWgamepadstate
  *  bundle, if present.  This can be disabled with the @ref
  *  GLFW_COCOA_CHDIR_RESOURCES init hint.
  *
+ *  @remark @x11 This function will set the `LC_CTYPE` category of the
+ *  application locale according to the current environment if that category is
+ *  still "C".  This is because the "C" locale breaks Unicode text input.
+ *
  *  @thread_safety This function must only be called from the main thread.
  *
  *  @sa @ref intro_init

src/x11_init.c

@@ -936,15 +936,12 @@ Cursor _glfwCreateCursorX11(const GLFWimage* image, int xhot, int yhot)
 
 int _glfwPlatformInit(void)
 {
-#if !defined(X_HAVE_UTF8_STRING)
-    // HACK: If the current locale is "C" and the Xlib UTF-8 functions are
-    //       unavailable, apply the environment's locale in the hope that it's
-    //       both available and not "C"
-    //       This is done because the "C" locale breaks wide character input,
-    //       which is what we fall back on when UTF-8 support is missing
+    // HACK: If the application has left the locale as "C" then both wide
+    //       character text input and explicit UTF-8 input via XIM will break
+    //       This sets the CTYPE part of the current locale from the environment
+    //       in the hope that it is set to something more sane than "C"
     if (strcmp(setlocale(LC_CTYPE, NULL), "C") == 0)
         setlocale(LC_CTYPE, "");
-#endif
 
 #if defined(__CYGWIN__)
     _glfw.x11.xlib.handle = _glfw_dlopen("libX11-6.so");