How to use a newly installed font as an input source

Go to System Preferences > Accounts > Login Options and tick "Show input menu in login window" you'll then be able to change it in the screen saver password window as well

In OS X 10.7 Lion, "Accounts" is called "Users & Groups". The setting "Show input menu in login window" really ought to be active by default when plural language input sources are enabled, though.

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.