The Jusos are part of an international organization (basically the youth organization of the Socialist International), called IUSY (International Union of Socialist Youth).
A flag of the IUSY is plain red with a hand grasping a rose outlined in white; over it "IUSY," under it the long name.
I believe the bottom inscription is intended to be Spanish — if so, it should read "Juventudes" not "Joventudes."
Of course this should read "Juventudes," but the actual flag samples the Munich Jusos sometimes use show erroneously "Joventudes." So I made the image accordingly.