You can get date and time format and the first week day options using user/get_locale command:

svc=user/get_locale&params={"userId":<long>}

If settings never changed via user/update_locale then blank object returned:

{}

If settings changed then response will be the following:

{
    "fd":<text>,   /* date and fime format */
    "wd":<uint>    /* the 1st week day: 1 - Monday, 7 - Sunday */
}

 More info about date and time format may be get from here.