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

Fixes #1587.
Fixes #1636.

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