Dictionary application doesn’t show all the dictionaries I can use

Many people have tried to find the files for Apple's spell check dictionaries but I don't recall any successes.

As for the Language tab in Language & Text, it really doesn't have anything to do with spelling as such, although the top item on the list can set the default spell check dictionary in some circumstances. The top item on the list also sets the OS and app language (for the 22 that are available), the default collation order, and the preferred language requested by Safari. The order of the list can determine the app language used if a localization for the top item is not available. The order can also determine whether a Japanese or Chinese font is used for text which is ambiguous in that regard. I think adding an item to the list can sometimes provide additional encodings in Mail app.

For OS X you can add 3rd party dictionaries. See

http://m10lmac.blogspot.com/2013/01/adding-dictionaries-to-dictionaryapp.html

Unfortunately it's not possible for iOS yet as far as I know.