Please contact the developer at mrwarper <whirly thing> yahoo dot es for bug reports and feature requests -- don't use the reviews for that!
1. If the switcher lists many pre-installed dictionaries you don't want in the menu
At least 3 workarounds have been used by different users:
1) Delete unused dictionaries from /usr/share/myspell/dicts (*.dic and *.aff files), and leave only the languages you use (*.dic and *.aff). [Backup before you delete anything.]
2) Remove symlinks from /usr/share/myspell
3) 1. Delete the dictionaries symbolic links in /usr/lib/thunderbird/ (from a command prompt, issue 'sudo rm /usr/lib/thunderbird/dictionaries')
2. Add a dpkg-divert rule to prevent link from being restored after each TB update ('sudo dpkg-divert --add /usr/lib/thunderbird/dictionaries').
3. Manually install the dictionaries you do want (Tools > Options or Edit > Preferences > Composition > Download more dictionaries).
The following localizations have been temporarily removed complying with Mozilla's policies because they are outdated:
ca-AD, da, hr, it, ko-KR, pt-BR, ru, sk, sv-SE, tr-TR, uk-UA, zh-CN, zh-TW. Old localizations and directions are available upon request to ease translation volunteering.
As of v1.3 includes and supersedes Dictionary Switcher for Thunderbird: https://addons.mozilla.org/addon/dictionary-switcher-for-thunde/
Switcher for Thunderbird should be removed before installing this version.