First, this error is not in current trunk of gnustep-base. I found it in 'reorg' branch
but not in trunk
You seem to be having an ANCIENT version of -base on your machine:
libgnustep-base.so.1.15
Having not updated recently, I have libgnustep-base.so.1.24.7. It is possible that the application is linking against and loading the wrong, ancient -base.
That said, I would also suggest you delete anything that's not your custom data. /etc/GNUstep/GNUstep.conf will contain a reference to what "system" and "local" domain directories are read by GNUstep. Unless they contain useful data (especially 'system' should not contain any, just like OS X's /System should not contain user modifications), nuke them.
Unless you do this, it's hard to guess what's going wrong. Certainly the gnustep-base version you have is unable to read the "user defaults" or, apparently, find the localizations. (Just glancing at the old code in reorg branch.) Why? Who knows?
The easiest way to get into a consistent state is to nuke all 'system' and 'local' (which are still global) directories. Apple treats 'system' as nukeable at OS upgrade on OS X. You should as well. (And treat 'local' as such for added safety.)
While at it, nuke /etc/GNUstep/GNUstep.conf and reinstall gnustep-make as well.