If there was a worldwide standard, then WMO would have imposed it to its members. Here are the definitions in the American (NOOA/NWS) standard:
Light rain: falling rate of less than 2,54 mm/h.
Moderate rain: falling rate of more than 2,8 mm/h, but less than 7.6 mm/h.
Heavy rain: fallin rate of more than 7.6 mm/h.
So clearly different standards compared to DWD. Nowadays, automated weather stations use rain gauges with sensors, so one by means of the computer inside the station has a clear and accurate computation of rain/snow falling rates.