I think it may be an isolated error, my sister's and my girlfriend's computers have the same instalation[1] with the same apt-policy and after I rebooted my system and corrected the error I upgraded their computer and the problem did not appear in any case.
If you are interested in it i can try to downgrade locales package and try the upgrade again to see what happen. I don't have the old package in /var/cache, but I've readed that there is now kind of archive with all versions of packages and I could find it. But if no one else has reported this ... anyway, if I can help debbugind the problem just ask me, this is the only way I can colaborate with debian project rigth now and I really want to (colaborate).
[1] In fact, mi sisters computer is the same model of laptop.