I know this discussion about spell checkers for Pages, here.
I am searching for the files ending with .aff or .dic.
Without them, you cannot use them in system.
My current OS X is Yosemite 10.10.2.
The manual of OS X says that in Settings > Keyboard > Spelling > Set up ...
To add an additional spelling dictionary, copy the dictionary files (.aff and .dic) to the Spelling folder in your Library folder.
Is there any standard dictionary for Finnish?
Best Answer
The Finnish language is not like others and the spell checking is more complicated than just .aff/.dic files. As Tom wrote, the solution is to use Voikko project's VoikkoSpellService (osxspell) which creates system wide spell checker for Finnish.
It works also on OS X Yosemite but the /Library folder is hidden and you have to make the Services folder if it doesn't exist. The readme in explains the rest steps to make it work.