What was the surprise that Duolingo announced on twitter for this week?
Was it the duolingo plus thing?
Twitter is more for their external-facing communications. They wouldn't use it to announce something entering the Incubator (like Cantonese).
If I remember correctly, didn't this:
Get ready to learn about lots of new “characters” on Duolingo. ;)
... ended up being this:
Surprise... we’re finally building a Japanese course! It’ll be ready in one month. Click here to get notified
(I think Japanese was in alpha at the time)