I think the answers above were good at the time of writing but a slightly confusing as they mention "set the region\language to a country that is [metric / imperial]". I am not sure this still exists...?
As documented in mid December 2018 in the ANAFI User Guide v2.3 and in the FreeFlight 6 App release 6.3.1 for iOS (and supposedly in v6.3.0 for Android, though I can not verify this) you can now set the Measurement System to Auto, Imperial or Metric.
To do so, launch the App, click on My.Parrot in the bottom left corner to check your App version is 6.3.1 (or higher if you read this at a later stage), click back to the home screen of the App, click the cog in the top right corner and scroll to the last Preference Option under "Interface" and make your selection
In the UK we are pretty used to metric measurements but not for speed. But alas in metric if gives you metres/sec and imperial feet/sec (or something like that). I am used to mph and omg neither of which seem to be an option