I did manage to fix this. Since Vector Linux upgraded to Thunderbird version 11, I was able to get the en-GB language pack from here
, which I installed similar to installing add-ons. I then opened the config editor in Thunderbird (Preferences -> Advanced -> General), and searched for the entry general.useragent.locale and set its value to en-GB. After restarting, the changes took place.
Since the developers have updated the thunderbird package, it might be a good idea to update the various language packs too. Currently language packs are all version 3.1.7, rather than version 11 (so they don't work with version 11).