So I just received my June challenge: double the movement (red) ring 12 times. Should be fairly easy.
So far my challenges have been:
10/2017 - Burn 20626 calories
11/2017 - Achieve an average of 64 minutes of exercise each day.
12/2017 - Double the movement ring 3 times
1/2018 - Achieve an average of 67 minutes of exercise each day.
2/2018 - Complete three exercises in a month
3/2018 - Achieve 240,6 km in a month (240 km of what?)
4/2018 - Double the movement ring 12 times.
5/2018 - Close all three rings 29 times.
6/2018 - Double the movement ring 12 times.
Some of these does not show the challenge but the result. Some was a walk in the park (e.g. complete three exercises in one month which I easily completed on day three), but others (like the 67 minute exercise average) were a lot harder.
As for the achievement/challenge interchangeability: My phone is set to Swedish. it says "Utmaning för juni" which literally translates into "Challenge for June", but the tab is called "Prestationer" which is "Achievements" Do phones set to English say challenge or achievement?