Another personal note….
Thunderbird does not adhere to any of the time or date format settings defined via KDE system settings. So, if you want to go for a 24 hour clock as used in Europe, a German keyboard and yet stick to the general English language you need to change the global locale settings of your machine, as e.g. described here.
Still easier, log into GNOME -> System -> Administration -> Language Support and set the format to English (United Kingdom) or English (Denmark) in the Text tab. Finally, use the Apply System-Wide button.