Lucid: http://www.secudb.de/~seuffert/mozilla/
Karmic:
- https://launchpad.net/~ubuntu-mozilla-daily/+archive/ppa/+packages ( Will install also firefox 3.5 daily build by default!! )
- http://blog.thomas-lauria.de/archives/9-Mozilla-Lightning-1.0b1-for-Thunderbird-3-on-linux-amd64.html
- Download Dutch 32 bit tar version from http://download.mozilla.org/?product=thunderbird-3.0&os=linux&lang=nl
- Extract tar
- cp thunderbird-3.0/dictionaries/nl.* /usr/lib/thunderbird-3.0*/dictionaries/
Update: