I have never touched the settings on my Nav V's I just leave it all in miles per hour. The conversion from kph to mph (80=50, 50=30 etc) is easy enough in your head, not least as the speed limits are not really so very different to ours, ie a town or village is probably always near enough 30 mph = 50 kph, unless signed otherwise. On the national limit roads, do just as you do here in the UK and ride at whatever speed you deem appropriate.
The vehicle's current speed versus the current road's speed limit is displayed by default on the dashboard and adjusts automatically. If anyone really must change ALL the settings and displayed speeds / distances from imperial to metric, then indeed they'll need to go into the device's settings screen.