A first, likely simpler solution can be achieved by GUI on some systems. Check for a menu like "language settings" and a point "supported languages". There you can remove whole languages in one go.
For more fine grained control (e.g. removing language dialects) see below.
A full solution can be found by combining the existing answers:
There are multiple locations for the languages (dictionaries):
- /usr/lib/thunderbird/dictionaries (which is just a symlink to /usr/share/hunspell)
(from Ringtail, Chris and Calimos answer)
These are system installed dictionaries. So to cleanly remove them you have to
sudo apt remove the packages that installed the files. To find out which packages those reside in you can use
apt-file search <path/to/file>.
Relevant packages are:
You can put all 3 into a
sudo apt remove, check the matches before confirming (WARNING: confirming it would remove ALL dictionaries which is usually not what you want) and abort. Then copy&past the relevant packages into a fresh command.
Alternatively start with e.g.
sudo apt remove hunspell-<TAB> and let autocompletion list the installed packages.
sudo apt remove hunspell-ru hyphen-ru mythes-ru
This would remove all russian language stuff.