Remove language dialects from Thunderbird

I was able to resolve this by uninstalling the respective hunspell packages:

First I searched for all installed hunspell packages:

aptitude search hunspell|grep ^i

Which returned this list:

i   hunspell-de-at   
i   hunspell-de-ch   
iB  hunspell-de-de   
i A hunspell-en-ca   
i A hunspell-en-gb   
i   hunspell-en-us   
i A hunspell-en-za   
i A libhunspell-1.3-0

Then I removed the unnecessary ones with:

sudo apt purge hunspell-de-at hunspell-de-ch hunspell-en-ca \
hunspell-en-gb hunspell-en-za

Note: A few redundancies still remain, which are caused by the fact that some packages install symlinks to other languages which then show up as duplicates in thunderbird. For example hunspell-de-de installs a symlink name de_BE.aff in /usr/share/hunspell which points to de_DE.aff. See https://launchpad.net/bugs/651586 (Thanks to Gunnar Hjalmarsson for pointing this out in the comments).

You can find the installed dictionaries at /usr/lib/thunderbird/dictionaries and delete the unwanted .aff and .dic dictionaries there.

Things change depending on Thunderbird and Ubuntu version so it's worth noting for future readers that I am on Ubuntu 14.04 and Thunderbird 38.8.0.