Hunspell is a spell checker and morphological analyzer library.
It is the spell checker of LibreOffice, OpenOffice.org, Mozilla Firefox 3 & Thunderbird, and more.
Two or three files are used for each language. You can get dictionary files from the Internet or from other people.
Hunspell only supports UTF encodings.
Additional description is available on the Hunspell home page, in Wikipedia, and so on. Here are some internet sites:
https://sourceforge.net/projects/hunspell/files/Hunspell/Documentation/ (click the README.txt or one of the *.pdf files).
https://www.libreoffice.org/, https://www.openoffice.org/, https://en.wikipedia.org/wiki/Firefox.
About spelling dictionary files