I have set up russian locale and run gtkballs, but interface is still in English. stracing the binary revealed that it looks for locale files under /usr/games/share/locale/ directory, but in fact *gmo files are installed to /usr/share/games/locale/. Further investigation shown that this is caused by a bug in gtkballs, which is triggered by the case if configure is run with --datadir other than $prefix/locale, which is exactly what Gentoo's games.eclass is doing. So, this is an upstream bug triggered by somewhat unusual (but still correct) flags to configure. I have already sent a patch fixing that to the gtkballs maintainer; still this is probably worth fixing in Gentoo as well. So, if you want to fix the bug here in Gentoo, I will attach a patch to gtkballs' configure.in and configure, as well as a patch for gtkballs ebuild.
Created attachment 58082 [details, diff] patch to fix localedir in configure.in and configure I chose to patch configure as well in order to avoid running autoconf.
Created attachment 58083 [details, diff] patch to ebuild adding 'epatch' to src_unpack()
thanks, gtkballs-3.1.5-r1 now in portage !