thunderbird in french

hello,
i bought an U3 usb key.
i download thunderbird for U3 by the U3 menu and i install the version 1.5.0.10 (20070221).
the version i have downloaded is in english and i want to have it in french.
i was on the thunderbird site and i have done all:
Downloading a language pack

To download a language pack:

1. Go to: http://releases.mozilla.org/pub/mozilla.org/
2. Go to the directory there for the Mozilla application you want to change—for example: thunderbird
3. Go to the releases directory.
4. Go to the directory for the release you are using—for example: Thunderbird 1.5.0.8
5. Go to the directory for your operating system—for example: win32
6. Go to the xpi directory there.
7. Install the xpi for the language, or language and region, that you want. For example, the French language pack is: fr.xpi

Note: To install a language pack in Thunderbird, do not click the link to the xpi file. Instead, open Thunderbird's Extensions or Add-ons window, then drag the link from your browser and drop it there. Alternatively, save the xpi file on your computer and install it in Thunderbird later.

when i click on tools / extensions i have this:
francais language pack 1.5.0.10

but thunderbird is still in english
what is wrong ?
thanks