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.
What I mean to say is that an English fallback is fine, but the current implementation, which returns the first result of the filter, should instead explicitly look for an English fallback instead of assuming that `agreements[0]` will be the English version.