Most of the dictionaries are symlinks from other basefiles. I have used these lines in my postinstall to remove dictionaries
echo "Remove excessive spell checking lists" cd /usr/share/myspell find . -type l -exec rm {} ; \rm -f de_AT.* de_CH.* en_CA.* ko_KR.*
That will give you a much smaller list of dictionaries in thunderbird.
Thanks a lot, exactly what I needed. It worked fine.