Create a small bash file

vi ~/thunder

and cut and paste the following in it.

export LC_TIME=en_GB 
exec /usr/bin/thunderbird "$@

Then make the file executable and edit the launcher

chmod 774 ~/thunder gnome-desktop-item-edit /usr/share/applications/mozilla-thunderbird.desktop

and replace the command with ~/thunder i.e. the path of the above file.

Note: unfortunately you will have to repeat the last command after each upgrade