In the system tray I can change the date format to dd/mm/yyyy, That is normal for my country. But in Thunderbird I see the date of messages displayed in the American format. MM/DD/YYYY.
It seems that Thunderbird takes the information from the short format in the system. But it is correct.
And I cannot add any languages in the locale. Netherlands is selected, but only US English is available when I click “Add language”.
Where did you set the locale? In Systeeminstellingen - Land/Regio Taal ?
Did you logout and back in after setting the locale’s. Thunderbird does not take this from some place, it takes it directly from the users locale settings.
If you pick “Nederlands” at install time, everything works well, that’s for sure.
In Yast - Systeem - /etc/sysconfigeditor search for LOCALE and see that they are all nl_NL or nl_NL.UTF-8
I think you missed something during install. Is Thunderbirds interface in dutch, by the way?