OS X spellchecker is essentially the open-source Hunspell engine. It uses *.aff and *.dic files as spelling dictionaries. This is the same format as used by OpenOffice and many other applications. You can either try to locate *.aff and *.dic files for your language somehow, or you can download the corresponding OpenOffice dictionary extension. That is an OXT file, which is essentially a zip with a custom file extension. Unzip it with a zip utility, and copy the dictionary files to ~/Library/Spelling. It should work.