If i18n message is not found, apply 'yy-mm-dd' to datepicker widget

fixes #1149 and all it's friends.

The format can be combinations of the following:

    d - day of month (no leading zero)
    dd - day of month (two digit)
    o - day of the year (no leading zeros)
    oo - day of the year (three digit)
    D - day name short
    DD - day name long
    m - month of year (no leading zero)
    mm - month of year (two digit)
    M - month name short
    MM - month name long
    y - year (two digit)
    yy - year (four digit)
    @ - Unix timestamp (ms since 01/01/1970)
    ! - Windows ticks (100ns since 01/01/0001)
    '...' - literal text
    '' - single quote
    anything else - literal text