Revise localized agreements

This is a very rough draft of revising the way we're localizing the agreements after the discussion following https://github.com/mozilla/fxoss/issues/199#issuecomment-51694614. Will utilize the "Ready for Review" label when appropriate.
I included the English fallback based on a conversation with @bensternthal and someone from legal-- for some languages, we may want to go live with a "partial translation" where we have some or all of the rest of the site translated, but not the download agreement.