Outdated language support packages not removed on upgrade

After upgrading to Oneiric I noticed that the "gnome" package was not installable because aptitude said there was an unresolvable dependency of language-support-translations-en on evolution-documentation-en. After looking around I discovered that language-support-translations-en was installed but not available in the archives! Evidently it previously was part of Ubuntu but became obsolete during an upgrade, but the upgrade tool never removed it. Using apt-show-versions, I discovered several more obsolete language packages:

$ apt-show-versions | grep -v oneiric
...
language-support-en 1:9.10+20090909 installed: No available version in archive
language-support-fonts-ja 1:10.04+20100420 installed: No available version in archive
language-support-input-ja 1:9.10+20090915 installed: No available version in archive
language-support-writing-en 1:10.04+20100311 installed: No available version in archive
language-support-writing-ru 1:10.04+20100311 installed: No available version in archive
...
$

The release upgrade tool should remove obsolete language packages to prevent dependency problems like this in the future.

Note that another user reported this same problem in comment #5 on bug 333401.