Hang about - I only noticed now, but I reckon the problem is the case-insensitive HFS+ filesystem on OSX... grrr... in a standard HFS+ filesystem acs-subsite.AR_EG.utf-8.xml and acs-subsite.ar_EG.utf-8.xml are the same file... so the files with capitalised country codes are in the CVS attic and shouldn't be getting checked out, but CVS is being stupid and getting them confused... so the next question is why isn't this happening on -HEAD or on the tip of -5-1?
I don't know why you aren't seeing it on oacs-5-1, but it is certainly caused by the fact that that the ALL CAPS versions of these files were removed, and correctly mixed case versions were added. If you aren't working directly with these files, it is safe to bulk-delete them and then retrieve them from CVS - that may clear out the problem.