I have not noticed that, thanks for the pointer. I did try other languages but I guess they are all not listed. I will do some more searching to figure out how to install it. Thanks.
Ok figured it out.
Add: de_DE.ISO-8859-1 ISO-8859-1 or de_DE.UTF-8 UTF-8
Run: dpkg-reconfigure locales
That worked for me. Further information found here
(German - wiki.ubuntuusers.de)
Edit: I did try the above again on a different Server with less luck when I remembered on the last Server I did also modify the german array in /usr/share/squirrelmail/functions/i18n.php and added de_DE.UTF-8 to it. see below for example.
I changed the following:
$languages['de_DE']['LOCALE'] = array('de_DE.ISO8859-1','de_DE.ISO-8859-1','de_DE');
to the following:
$languages['de_DE']['LOCALE'] = array('de_DE.ISO8859-1','de_DE.ISO-8859-1','de_DE','de_DE.UTF-8');
Now it should definetly work sorry for that. (Your "Inbox" should now be "Posteingang" or whatever language you added above.)