Hunspell - Can't open affix or dictionary files for dictionary named en_US

Execute hunspell -D. You should get output like this:

.::/usr/share/hunspell:/usr/share/myspell:
/usr/share/myspell/dicts:/Library/Spelling:

AVAILABLE DICTIONARIES (path is not mandatory for -d option):
/Library/Spelling/en_GB
LOADED DICTIONARY:
/Library/Spelling/en_GB.aff
/Library/Spelling/en_GB.dic

This lists the directories in which hunspell is searching for dictionary files, as well as the dictionaries is has found. If the dictionary en_US isn't listed, you haven't got that particular dictionary installed.

To install a dictionary, search for it in the LibreOffice extension repository. Download it then extract the .aff and .dic files to one of the locations listed by hunspell -D. For example:

# First download dict-en.oxt
unzip dict-en.oxt -d dict-en
cp dict-en/en_GB.aff dict-en/en_GB.dic ~/Library/Spelling/
rm -r dict-en