Thanks to Carnotzet.
According to this post, Soviet battleships are planned to be added next year.
Now, we don’t know if it concerns a full branch or only a few premium ships (even if the formulation seems to point at the former).
Please be cautious when taking this information for granted since plans can always be altered. It is but one statement made by one developer and is not to be taken as an official announcement. I know how plans for new branches can get people all worked up (especially when the new branch happens to be a Soviet one) so please don’t jump the gun and start a flame war for nothing. Besides, we have no information on the other branches WG is planning to release next year. Thank you.