/usr/share/i18n/locales/de_DE is pretty outdated - it still has the old German date format ("DD.MM.YYYY"). GNU's glibc-localedata-2.0.6.tar.gz seems to have it right since at least 1997-12-28. Pristine sources? :-) Easy solution: Edit de_DE and re-run localedef. But the right way (TM) should of course be modifying the .src.rpm.
I am told by the glibc maintainers that there has not been any official decision on this, so the format glibc uses is still the active one.
In localedata/locales/de_DE (timestamped: 1997-03-05) from <URI:ftp://ftp.gnu.org/gnu/glibc/glibc-localedata-2.0.6.tar.gz> (timestamped: 1997-12-29), line #2200 says: | d_fmt "<%><Y><-><%><m><-><%><d>" That seems to be an "official" decision by the glibc maintainers, and it is also perfectly correct according to EN 28601 and DIN 5008 which apply to the Federal Republic of Germany (cf. <URI:http://www.cl.cam.ac.uk/~mgk25/iso-time.html>). I may be missing some point.
Please report it to the glibc maintainers. this is not the kind of change we can do unilaterally.