Probably it should prefer whatever scale is used locally, but allow conversion to others? (In the same way that OSM uses feet and inches in the US and meters in other countries.)
Here is an equivalent discussion for climbing grades:
https://wiki.openstreetmap.org/wiki/Talk:Climbing#Prefered_grading_system
In the US there’s the Yosemite Decimal System (YDS) (which is used in climbing:grade:yds_class but also applies to trails) and apparently there are different scales for Shenandoah National Park, Willamette National Forest, etc.
There’s an attempt at a conversion chart here: Hiking/Scrambling Grade Conversion Chart : General
I wholly disagree with the idea that we shouldn’t have difficulty/visibility/smoothness/solidness ratings just because they’re “subjective”. For comparison, climbing route grades are subjective, and people may disagree about the exact number, but they’re absolutely necessarily for safe climbing and planning.