I like having the user interface of my Linux Mint 19.1 in English, but the regional settings such as numbers and currency in a different language such as Swiss German.
Unfortunately, the date and time settings are set as part of the Language Settings and can not be controlled individually. As a consequence, the Lightning Calendar extension in Thunderbird displays the time as a.m / p.m instead of the desired 24h time format.
Override the system setting of the time format by adding the following two lines to your .profile file:
LC_TIME=de_CH.UTF-8 export LC_TIME
You can find the text file .profile in you home folder (enable hidden files in Nemo first).