Hello.
I configured metric preferences but in some screens, I’m notified with MPH equivalent.
For example, this week-end I got a new accomplishment over 50MPH whereas I ride at about 80KPH.
I think this is just a display variable error and not really a bug !
Marc, a new french addict 
It looks like this is your first time posting on the Zwift Forums. Welcome!
I had a look at our related article for general cycling achievements/badges, and it seems there are a few different MPH achievements, but only one specifically for KPH.
For example, the MPH badges like Whoa Nelly (Break 30 mph), Speed Demon (Break 40 mph), and Daredevil (Break 50 mph) don’t as yet appear to have a KPH equivalent, so it’s likely you just got one or more of those badges instead.
There is an extra credit achievement for “Ride at 100 kph (62.1 mph)”, however.
I’m sorry for the confusion, but I hope that helps!