[i18n] Support non-English banners with English fallback

[chalin]

Add support for homepage banner announcements.

Originally posted by @windsonsea in #2402 (comment)

Could we provide a placeholder-alike? If no localized zh text, use the en banner; if localized, use the zh banner.

@svrnm wrote:

Yes, having the English banner would be great. They are often short lived but important, so providing a translation everytime might not be possible

[chalin]

I have some ideas about how to implement this with Hugo's base features (rather than writing custom Hugo code). I'll probably work on this after I fix the canonical links issue