I know this discussion about spell checkers for Pages, here.
I am searching for the files ending with
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?