language-support-de can not be installed

What is the output of `LC_ALL="C" apt-cache policy language-support-de` and what happens if you try to install that package via apt-get or aptitude (via the console, rather than a GUI)? Can you give any more information about possible conflicts, please?

Hope the information below helps. I still cannot get a German keyboard layout with gnome. In a tty all works well, but not within the GUI.

Controlcenter->keyboard is set to german, keyboard is Generic 105 (Intl) PC

juergen@arbeitsplatz:~$ LC_ALL="C" apt-cache policy language-support-de
language-support-de:
  Installed: 1:8.04+20080214
  Candidate: 1:8.04+20080214
  Version table:
 *** 1:8.04+20080214 0
        500 http://de.archive.ubuntu.com hardy/main Packages
        100 /var/lib/dpkg/status
W: Duplicate sources.list entry http://archive.canonical.com hardy/partner Packages (/var/lib/apt/lists/archive.canonical.com_ubuntu_dists_hardy_partner_binary-i386_Packages)

juergen@arbeitsplatz:~$ sudo LC_ALL="C" apt-get install language-support-de
Reading package lists... Done
Building dependency tree
Reading state information... Done
language-support-de is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
W: Duplicate sources.list entry http://archive.canonical.com hardy/partner Packages (/var/lib/apt/lists/archive.canonical.com_ubuntu_dists_hardy_partner_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

juergen@arbeitsplatz:~$ sudo LC_ALL="C" apt-get update
Hit http://security.ubuntu.com hardy-security Release.gpg
Hit http://archive.canonical.com hardy Release.gpg
Hit http://security.ubuntu.com hardy-security Release
Hit http://archive.canonical.com hardy Release
Get:1 http://de.archive.ubuntu.com hardy Release.gpg [191B]
Hit http://de.archive.ubuntu.com hardy-updates Release.gpg
Get:2 http://de.archive.ubuntu.com hardy Release [65.9kB]
Hit http://security.ubuntu.com hardy-security/main Packages
Hit http://archive.canonical.com hardy/partner Packages
Hit http://security.ubuntu.com hardy-security/restricted Packages
Hit http://security.ubuntu.com hardy-security/universe Packages
Hit http://de.archive.ubuntu.com hardy-updates Release
Hit http://security.ubuntu.com hardy-security/multiverse Packages
Hit http://security.ubuntu.com hardy-security/main Sources
Hit http://security.ubuntu.com hardy-security/restricted Sources
Hit http://security.ubuntu.com hardy-security/universe Sources
Hit http://security.ubuntu.com hardy-security/multiverse Sources
Get:3 http://de.archive.ubuntu.com hardy/main Packages [1190kB]
Get:4 http://de.archive.ubuntu.com hardy/restricted Packages [7908B]
Get:5 http://de.archive.ubuntu.com hardy/universe Packages [4310kB]
Get:6 http://de.archive.ubuntu.com hardy/multiverse Packages [175kB]
Get:7 http://de.archive.ubuntu.com hardy/main Sources [338kB]
Get:8 http://de.archive.ubuntu.com hardy/restricted Sources [1646B]
Get:9 http://de.archive.ubuntu.com hardy/universe Sources [1325kB]
Get:10 http://de.archive.ubuntu.com hardy/multiverse Sources [60.2kB]
Hit http://de.archive.ubuntu.com hardy-updates/main Packages
Hit http://de.archive.ubuntu.com hardy-updates/restricted Packages
Hit http://de.archive.ubuntu.com hardy-u...

> W: Duplicate sources.list entry http://archive.canonical.com hardy/partner Packages
> (/var/lib/apt/lists/archive.canonical.com_ubuntu_dists_hardy_partner_binary-i386_Packages)

You should probably clean up your /etc/apt/sources.list to remove redundant lines and run `apt-get update` afterwards.
I do not see why Synaptic says that this package could not be installed as it is already installed according to apt-cache.

I removed some duplicate entries from the list. Then I did a Settings->Keyboard->Add (German), ->Remove USA.
Seems as if I have a German keyboard layout under Gnome now. So I think this could be called solved.

Strange thing was that I could no longer check the Default Radiobutton in the Keyboard settings.