The aggressive pruning of locales causes X11 input methods to fail in unexpected ways. For example, the library https://github.com/lv2/pugl calls XOpenIM in libx11 which returns NULL if any of the files C, compose.dir, or iso8859-1 are missing from /usr/share/X11/locale. This ultimately leads to a SIGSEGV when this NULL finds its way to a call to XUnsetICFocus. Perhaps pugl should be more careful (I've submitted a PR https://github.com/lv2/pugl/pull/83 ), but adding these files to the archlinux image could help other programs.
In this example program, XOpenIM returns NULL in the current archlinux images, but returns a reasonable pointer with this patched image.
/* cc xtest.c -lX11 */
#include <stdio.h>
#include <X11/X.h>
#include <X11/Xlib.h>
int
main()
{
Display *display = XOpenDisplay(NULL);
XIM xim = XOpenIM(display, NULL, NULL, NULL);
printf ("xim %p\n", xim);
return (0);
}