The modular tree puts the libX11 locale data files in /usr/lib/X11/locale, however these files are architecture independent data files, which should be installed in /usr/share on FHS compliant systems. The default location for these files for FHS compliant Linux (or other) installations should be /usr/share/X11/locale however it probably makes sense that this should be overrideable via ./configure options as well. Filed upstream as: https://bugs.freedesktop.org/show_bug.cgi?id=5064
Fixed in libX11-0.99.3-4 in rawhide.
Note: A much more complete patch supplied by Daniel Stone is in the upstream bug report linked to above. Daniel's patch fixes things to also work with the modular i18n modules, however we are no longer shipping them, as they're built directly into our X server by default to simplify things.