Ubuntu 18.04 comes with a lot of variations of English, which can clutter the spellchecker in Thunderbird and Firefox.
A quick way of removing some of the variations of English, the safest method is uninstalling the unneeded language packages.
In my case, I had these in my /usr/share/hunspell folder:
da_DK.aff da_DK.dic en_AU.aff en_AU.dic en_CA.aff en_CA.dic en_GB.aff en_GB.dic en_US.aff en_US.dic en_ZA.aff en_ZA.dic
To uninstall the unneeded packages, only keeping Danish and US English:
$ sudo apt purge hunspell-en-au hunspell-en-ca hunspell-en-gb hunspell-en-za
Result:
da_DK.aff da_DK.dic en_US.aff en_US.dic
