Thunderbird
Installing Spellcheck Languages
http://www.mozilla.com/thunderbird/dictionaries.html
The Thunderbird spell checker only comes with a US English dictionary installed by default. However, many other dictionaries are widely available.
Mozdev XPI Packaged Dictionaries
You can download additional dictionaries from the Mozdev project at http://dictionaries.mozdev.org/. Instructions for installing the dictionaries are found here.
Since the dictionaries are installed to the system myspell directory, ensure that there are proper read permissions for the intended users. If you installed Thunderbird with --prefix=/usr, the following commands will set the correct permissions for all users.
chmod -v 644 /usr/lib/thunderbird-1.5/components/myspell/*
OpenOffice ZIP Packaged Dictionaries
The dictionaries used in the Mozilla spell checker come from OpenOffice Lingucomponent Project. Alternatively, you can download the dictonaries directly from here. These dictionaries come in zip format and are often newer than the versions available from Mozdev.
To install these dictionaries, you will need the unzip utility. For example, to install the en_GB.zip dictionary, you can use these commands.
unzip -d /usr/lib/thunderbird-1.5/components/myspell en_GB.zip
Again, ensure that the permissions are correct for your system. As noted on the Mozilla site, Thunderbird will not list the language and country of your installed dictionary using the file names of the OpenOffice zip files. Change the names of the installed files with this for loop:
cd /usr/lib/thunderbird-1.5/components/myspell
for F in *_*.{dic,aff}; do
mv -v $F ${F//_/-}
done
