After an upgrade, my Thnderbird started to use a weird US-date format and forgot my setting for this. I did not find any setting either. But this seems to work fine:
Create a shell script
#!/bin/bash
export LC_TIME=sv_SE.utf8
export LC_DATE=sv_SE.utf8
exec /usr/bin/thunderbird
and make sure that this gets called when thunderbird is started instead of directly /usr/bin/thunderbird.