More on this bug. I found why this happens.

This is the system without italian localization

giovanni@shrimp:~$ dpkg-query -l 'thunderbird*'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Nome                                    Versione                                Descrizione
+++-=======================================-=======================================-==============================================================================================
ii  thunderbird                             3.1.9+nobinonly-0ubuntu3                mail/news client with RSS and integrated spam filter support
ii  thunderbird-globalmenu                  3.1.9+nobinonly-0ubuntu3                Unity appmenu integration for Thunderbird
un  thunderbird-gnome-support               <none>                                  (nessuna descrizione disponibile)
un  thunderbird-locale-en-gb                <none>                                  (nessuna descrizione disponibile)
un  thunderbird-locale-en-us                <none>                                  (nessuna descrizione disponibile)
un  thunderbird-locale-it                   <none>                                  (nessuna descrizione disponibile)
un  thunderbird-thunderbird-locale-en-gb    <none>                                  (nessuna descrizione disponibile)
un  thunderbird-thunderbird-locale-en-us    <none>                                  (nessuna descrizione disponibile)
un  thunderbird-thunderbird-locale-it       <none>                                  (nessuna descrizione disponibile)
giovanni@shrimp:~$

And this is the setting of the launcher. You can see the /home/giovanni/.local/share/applications/thunderbird.desktop desktop file, that contains the quicklist.

giovanni@shrimp:~$ gsettings get com.canonical.Unity.Launcher favorites
['nautilus-home.desktop', 'google-chrome.desktop', '/home/giovanni/bin/unity/CustomSSH.desktop', '/home/giovanni/.local/share/applications/thunderbird.desktop', 'gedit.desktop', 'virtualbox-ose.desktop', 'filezilla.desktop', 'libreoffice-writer.desktop', 'libreoffice-calc.desktop', 'ubuntu-software-center.desktop']

I removed Thunderbird icon from launcher, then installed missing language packages

giovanni@shrimp:~$ dpkg-query -l 'thunderbird*'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Nome                                    Versione                                Descrizione
+++-=======================================-=======================================-==============================================================================================
ii  thunderbird                             3.1.9+nobinonly-0ubuntu3                mail/news client with RSS and integrated spam filter support
ii  thunderbird-globalmenu                  3.1.9+nobinonly-0ubuntu3                Unity appmenu integration for Thunderbird
un  thunderbird-gnome-support               <none>                                  (nessuna descrizione disponibile)
ii  thunderbird-locale-en-gb                1:3.1.7ubuntu1                          Thunderbird English language/region package
ii  thunderbird-locale-en-us                1:3.1.7ubuntu1                          Thunderbird English language/region package
ii  thunderbird-locale-it                   1:3.1.7ubuntu1                          Thunderbird Italian language/region package
un  thunderbird-thunderbird-locale-en-gb    <none>                                  (nessuna descrizione disponibile)
un  thunderbird-thunderbird-locale-en-us    <none>                                  (nessuna descrizione disponibile)
un  thunderbird-thunderbird-locale-it       <none>                                  (nessuna descrizione disponibile)
giovanni@shrimp:~$

Logout and login, Added Thunderbird to launcher and the thunderbird.desktop file is not here, while the icon is in the launcher and I pressed "Stick on launcher"

giovanni@shrimp:~$ gsettings get com.canonical.Unity.Launcher favorites
['nautilus-home.desktop', 'google-chrome.desktop', '/home/giovanni/bin/unity/CustomSSH.desktop', 'gedit.desktop', 'virtualbox-ose.desktop', 'filezilla.desktop', 'libreoffice-writer.desktop', 'libreoffice-calc.desktop', 'ubuntu-software-center.desktop']
giovanni@shrimp:~$



Now you can see the difference, and why I can't see 

giovanni@shrimp:~$ diff -u /home/giovanni/.local/share/applications/thunderbird.desktop /usr/share/applications/thunderbird.desktop 
--- /home/giovanni/.local/share/applications/thunderbird.desktop	2011-04-19 06:54:44.068614817 +0200
+++ /usr/share/applications/thunderbird.desktop	2011-04-11 02:23:53.000000000 +0200
@@ -58,15 +58,3 @@
 GenericName[ug]=ئېلخەت دېتالى
 GenericName[zh_CN]=邮件新闻客户端
 GenericName[zh_TW]=郵件用戶端
-
-
-X-Ayatana-Desktop-Shortcuts=ComposeUL;ContactsUL
-[ComposeUL Shortcut Group]
-Name=Compose a New Message
-Exec=thunderbird -compose
-OnlyShowIn=Unity
-
-[ContactsUL Shortcut Group]
-Name=Contacts
-Exec=thunderbird -addressbook
-OnlyShowIn=Unity
giovanni@shrimp:~$