3 New additional flags were added in 0.5.12, all of which were not mentioned in patch notes, so identification is tricky.
First Flag: “Skull III” flag, presumably related to ranked battles, allthough the next season being the 5th season, it is unclear if this flag is related:
EDIT: This flag is recieved upon achieving Rank 1 a fourth time.
Second Flag, completely un-identified, I have no idea what is is for. Russian star suggests it is russia related, Russian holiday? Leningrad exclusive Flag? something Else? Unknown:
Third Flag: I am 95% certain this will be the exclusive flag for the HMS Perth, which will come with a bundle that does nothing aside to prove you have money;
EDIT: I was completely wrong, it is the commemorative flag for reaching rank 15 within season 5 or ranked.