`Locale.MeasurementSystem`

ContentView.swift

struct ContentView: View {
    @Environment(\.locale) var locale

    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundStyle(.tint)
            Text("Locale: \(locale.identifier)")
            Text("Measurement: \(locale.measurementSystem.identifier)")
            Text("3 miles: \(Measurement(value: 3, unit: UnitLength.miles), format: .measurement(width: .wide, usage: .road))")
        }
        .padding()
    }
}

test.swift

import Foundation

// https://developer.apple.com/documentation/foundation/locale/4020222-measurementsystem
// "if the user overrode the default measurement system, this property provides the user’s preference."
print(Locale.current.measurementSystem) // us, uk, metric (will be metric in a playground)

let en_GB = Locale(languageCode: .english, languageRegion: .unitedKingdom)
let en_US = Locale(languageCode: .english, languageRegion: .unitedStates)
let en_DE = Locale(languageCode: .english, languageRegion: .germany)

var components = Locale.Components(languageCode: .english, languageRegion: .unitedStates)
components.measurementSystem = .metric // override
let en_US_metric = Locale(components: components)

let m = Measurement(value: 3, unit: UnitLength.miles)

print(m.formatted(.measurement(width: .wide).locale(en_US))) // 3 miles
print(m.formatted(.measurement(width: .wide).locale(en_GB))) // 3 miles
print(m.formatted(.measurement(width: .wide).locale(en_DE))) // 4,8 kilometres
print(m.formatted(.measurement(width: .wide).locale(en_US_metric))) // 4.8 kilometers

print(m.formatted(.measurement(width: .wide, usage: .visibility).locale(en_US))) // 3 miles
print(m.formatted(.measurement(width: .wide, usage: .visibility).locale(en_GB))) // 3 miles
print(m.formatted(.measurement(width: .wide, usage: .visibility).locale(en_DE))) // 4.8 m
print(m.formatted(.measurement(width: .wide, usage: .visibility).locale(en_US_metric))) // 4.8 kilometres

print(m.formatted(.measurement(width: .wide, usage: .asProvided).locale(en_US))) // 42 miles
print(m.formatted(.measurement(width: .wide, usage: .asProvided).locale(en_GB))) // 42 miles
print(m.formatted(.measurement(width: .wide, usage: .asProvided).locale(en_DE))) // 42 miles
print(m.formatted(.measurement(width: .wide, usage: .asProvided).locale(en_US_metric))) // 42 miles