The 大漁旗 naval flag is awarded if you collect the two decals and the 3D decoration in the active DMM event (【実録】元海上自衛隊員による日本語ボイス（艦艇乗組員）新収録！ゲーム内実装記念キャンペーン - War Thunder （ウォーサンダー）- DMM GAMES). I have collected them and received the flag. When trying to apply the flag an error “Not available without an achievement” is shown.
There is a post on the old forum about a similar issue which was resolved after playing a game, but so far this has not fixed it.
