allow additional i18n formats to define defaults

a5f62d96b124564f522eac7925e9c3d22c37e503 would had liked to let
usage="DATE_TIME" formatindex="55" define the default format of that
usage, which wasn't possible. There is no reason to prevent this.

Change-Id: Ibef453c8a07aa03b0a8bb626b9cf71d362be0699
let YYYY/M/D H:MM be the default datetime format

Change-Id: I1d8af400810b9ffd4586320c706a1f00ef1c1369