Hello Erik,
That is because right now only the translations for testing and sid are updated. Translations for stable and oldstable stay the same (see ftp://ftp.debian.org/debian/dists/squeeze/main/i18n/). Either it should be impossible to translate old package descriptions or the translations for stable and oldstable should be updated as well. Thanks for looking into this problem, so translators won't do useless work :)
you are not alone: 632745. Cheers, Martin