New 0.15 can't find locales in /usr/share/locale because they actually placed in /usr/share/games/widelands/usr/share/games/widelands/locale. There patch to fix this error.
Created attachment 254925 [details, diff] widelands-0.15-locale-fix.patch
Created attachment 254927 [details, diff] widelands-0.15.ebuild.patch Patch for ebuild.
in portage. thanks for the bug report and patch.