some time ago, Storm said there would be three branches this year. The French are already out and Storm confirmed the Japanese heavy tanks coming. Now, Anton "Evilly" Pankov (WoT producer) confirmed the identity of the third branch to come this year during the "Portraits of Victory" history-themed meeting with players (can be seen here at 5:40)
It will be the Czechoslovak branch.
According to Evilly, it will be a part of the EU tree (it's possible this is just obsolete info) and it will come by the end of the year. You can learn about the main branch composition here.