It's very anoying that the dictionaries from the Thunderbird Add-ons are not available for the version of Thunderbird from CentOS-5.
After getting some error messages, and trying around I found Localized Dictionaries for Mozilla Thunderbird on mozilla.org. Unfortunately the downloadable .xpi does not work for me?! However there is a nice note about OpenOffice.org Dictionaries and Thunderbird on the same page. This seems to work perfectly!