It uses the NSDateFormatString definition from the locale:

https://github.com/inverse-inc/sogo/blob/master/UI/MainUI/German.lproj/Locale#L10

For the timezone, my guess is that it uses your system's locales. What's the output of "localectl list-locales"?

I think I found the problem. I can't change the long time format in the preferences. For some reason it is set to "weekday, month, day, year". When I change it, click on "save", switch to the mail view and come back again the old setting is still there.

Log file when saving the settings:

12.4.2019, 00:17:58 "POST /SOGo/so/user@domain.tld/Preferences/save HTTP/1.0" 200 0/3677 0.364 - - 0
12.4.2019, 00:17:58 <0x0x55eaf17a2860[SOGoSieveManager]> WARNING: Could not delete Sieve script - continuing...: {RawResponse = "{ok = 0; }"; result = 0; }
12.4.2019, 00:17:58 <0x0x55eaf19b51c0[NGSieveClient]> TLS started successfully.
12.4.2019, 00:17:58 <0x0x55eaf1aaca50[NGSieveClient]> TLS started successfully.

I'm running SOGo inside a docker container. I changed the locale to de_DE.UTF-8 but that didn't change anything.

root@ed1297999e73:/# locale
LANG=de_DE.UTF-8
LANGUAGE=de_DE.UTF-8
LC_CTYPE="de_DE.UTF-8"
LC_NUMERIC="de_DE.UTF-8"
LC_TIME="de_DE.UTF-8"
LC_COLLATE="de_DE.UTF-8"
LC_MONETARY="de_DE.UTF-8"
LC_MESSAGES="de_DE.UTF-8"
LC_PAPER="de_DE.UTF-8"
LC_NAME="de_DE.UTF-8"
LC_ADDRESS="de_DE.UTF-8"
LC_TELEPHONE="de_DE.UTF-8"
LC_MEASUREMENT="de_DE.UTF-8"
LC_IDENTIFICATION="de_DE.UTF-8"
LC_ALL=de_DE.UTF-8