Add the hours attribute to your locale identifier (e.g., "en_US@hours=h23") with one of these values to get what you want:
h12
Hour system using 1–12; corresponds to 'h' in patterns. The 12 hour clock, with midnight starting at 12:00 am. As used, for example, in the United States.
h23
Hour system using 0–23; corresponds to 'H' in patterns. The 24 hour clock, with midnight starting at 0:00.
h11
Hour system using 0–11; corresponds to 'K' in patterns. The 12 hour clock, with midnight starting at 0:00 am. Mostly used in Japan.
h24
Hour system using 1–24; corresponds to 'k' in pattern. The 24 hour clock, with midnight starting at 24:00. Not used anywhere.