Can't change OS X Maps to use km instead of miles
If you want to change the system to use metric units, you have to select a region (locale), which uses only metric units.
Apple is not very good when it comes to internationalization. Especially the many small nationalities suffer from this attitude. It feels like American culture is walking all over you.

Go to System Preferences / Language &amp; Region / Select a region which definitely uses the metric system. Maps should finally work.