Comment 17 for bug 312483

To make it work edit /etc/environment (e.g. sudo emacs /etc/environment) and
add the following line to it:

LC_TIME="en_GB.UTF-8"

Save the file, log out and log in again. You can check that the variable has
been successfully set by opening a terminal and typing:

echo $LC_TIME

which should return:

en_GB.UTF-8

Now start Thunderbird again and hopefully all will be well. I know it is
annoying to have to do this but maybe the bug should be better addressed by the
creators of distributions, who fail to set LC_TIME as a default in
/etc/environment.