Duolingo has just released Latin as a language to learn. Here is the flag they ended up using: 
[![flag for latin][1]][1]

Here how it looks in the app:
[![Screenshot from Duolingo app][2]][2]


  [1]: https://i.stack.imgur.com/B6rxN.jpg
  [2]: https://i.stack.imgur.com/aaa3q.jpg