I do not know where exactly you see dates in a wrong format, since I see them mostly in a verbose format, and not in the numeric fomat, on the GC Mobile app, but the numeric date format can be changed in GC Web at https://connect.garmin.com/modern/settings. Since I do not know where you see it on the app, I cannot test whether the format then applies to the app too.
garmon connect seems fine.
it is the garmin golf app, and in the scorecards, it lists them by mm/dd/yy.
everything else on my phone/laptop/ipad is in format dd/mm/yy.
the fault clearly lies within the garmin app. it does not even give the option to choose the date format you would like