Important: Language packages need to be downloaded locally, and then opened in Mozilla Thunderbird. If you're using a Gecko compatible browser, DO NOT directly click on the following link, as it will install the lang pack in your browser rather than in Mozilla Thunderbird.
To have the user inteface language in your favourite language:

1. Thunderbird releases archive
2. In Mozilla Thunderbird, from the menu use choose Tools | Extensions. Click on «Get New Extensions» and browse for the local .XPI file.
3. Once the installation has completed sucessifully, restart the program from the command line prompt, adding the following switches: