These are hunspell dictionaries.
The basic requirements are .dic and .aff files. hyphenation files are prefixed with hyph_ for instance hyph_en-us.dic. Thesauraus files have an extension of .dat.
There are two sets of dictionaries in this repository, one is just pure hunspell dictionaries, the other are dictionaries from Open Office and Mozilla / Firefox.
Each directory has it's own install script which will install all the dictionaries it has in /Library/Spelling/.
The hunspell dictionaries are a simple zip file with a .dic, a .aff and a description.
Open Office uses an extension of .oxt for it's dictionaries and Mozilla uses .xpi.
Both types of files are actually zip files, change the extension to zip, unzip it and find the .dic and .aff files inside..
Open office dictionaries are available here. [open office dictionaries] (http://extensions.openoffice.org/en/search?f[0]=field_project_tags%3A157)
Mozilla / Firefox dictionaries are here. [Mozilla dictionaries] (https://addons.mozilla.org/en-US/firefox/language-tools/)
The only real difference beyond that is that the xpi files keep the dictionaries in a sub folder named dictionaries.
The install script accommodates both kinds of files and installs the dictionaries in /Library/Spelling/ the default location for dictionaries on OS X. Alternatively they can go in ~/Library/Spelling/ but why bother ?
If you only wish to install a few dictionaries, unzip them and copy the .dic and .aff files to /Library/Spelling/.