Determining which Locales use miles for distance formatting.

Locale+DistanceUnit.swift

import Foundation
import MapKit

extension Locale {
    
    var usesMilesForDistanceFormatting: Bool {
        guard usesMetricSystem else { return true }
        
        let miles = Measurement(value: 42, unit: UnitLength.miles)
        let kilometers = miles.converted(to: .kilometers)
        
        let numberFormatter = NumberFormatter()
        numberFormatter.locale = self
        
        let formattedMiles = numberFormatter.string(
            from: NSNumber(value: miles.value)
        ) ?? "42"
        
        let formattedKilometers = numberFormatter.string(
            from: NSNumber(value: kilometers.value)
        ) ?? "68"
        
        let meters = miles.converted(to: .meters)
        
        let distanceFormatter = MKDistanceFormatter()
        distanceFormatter.locale = self
        
        let formattedDistance = distanceFormatter.string(
            fromDistance: meters.value
        )
        
        if formattedDistance.contains(formattedMiles) {
            return true
        } else if formattedDistance.contains(formattedKilometers) {
            return false
        } else {
            assertionFailure("Could not determine if this locale uses miles.")
            return false
        }
    }
    
}

This updated version includes locales that don’t format miles using the same numerals as English. The locales that use miles for distance formatting are:

Burmese (Myanmar [Burma]), Cornish (United Kingdom), Spanish (United States), Vai (Latin, Liberia), Cherokee (United States), Vai (Latin), English (Myanmar [Burma]), Cherokee, Lakota, Scottish Gaelic (United Kingdom), English (United States, Computer), English (Liberia), Kpelle (Liberia), English (United Kingdom), Hawaiian (United States), Vai (Liberia), Fulah (Liberia), Vai, English (United States), Vai, Hawaiian, Welsh (United Kingdom), Lakota (United States), Kpelle, Burmese, and English

The locales that return true for usesMetricSystem and also use miles for formatting are:

Cornish (United Kingdom), Scottish Gaelic (United Kingdom), English (United Kingdom), and Welsh (United Kingdom)