The extensions are built using HFST — specifically hfst-ospell (source code) — and Voikko, specifically the LibreOffice-voikko extension (source code). The exact list of libraries and their versions used for the released spellers can be found on a separate page.
The spellers here are a direct application of the normative fst used as a spelling checker (in combination with an error model to create suggestions). This is work in progress, and there are several open issues still to be worked out. Although the available spellers and extensions should work fine for the specified LibreOffice versions, please bear with us and report any issues you find to firstname.lastname@example.org
Source code access
The source code repository for each language is found behind the links after each download link, and instructions for how to build each language can be found by following the instructions here.