Quick Answer:
Recompile the editor. Everything should work as intended.
The long answer:
The issue is with the glibc update
tar -tf /var/cache/pacman/pkg/glibc-2.25-7-x86_64.pkg.tar.xz 2>/dev/null| grep xlocal
usr/include/xlocale.h
vs
tar -tf /var/cache/pacman/pkg/glibc-2.26-4-x86_64.pkg.tar.xz 2>/dev/null| grep xlocal
Not sure how why and when they decided to start dropping header files from glibc but hey, warning would be nice.
Not an UE4 Issue per se, but more of an bleeding edge issue.
By the way, there are several bugs created for this 1464244 – XS modules fail to build because <xlocale.h> does not exist
Maybe a PR for the engine is in order as this seems to be a trend to remove the xlocale.h and would affect other distributions soon enough
Here is the glibc commit:
https://sourceware.org/git/?p=glibc.git;a=commit;h=f0be25b6336db7492e47d2e8e72eb8af53b5506d