I don't know if you can set individual files or not. You can however, set up a key sequence that will flip between languages though.
Go to regional and language options in the control panel, languages tab, details, key settings. You should be able to enter a hotkey sequence to switch between Spanish/English input.
Hope that helps.
Yup I agree completely with that, but you'll still have to also set the language in Word if you want the proofing tools to work.
I would create a simple macro that does the following actions in a file you want set for Spanish
Tools, Language, Set Language (Depending on your version)
You could record that macro and assign it to a toolbar button for easy access (or give it a shortcut key as well).
Are you Folding for a cure? Why not? Its easy, free, and doesn't hurt anyone - in fact it can only help!