Add a preferredLocales dependency

[davdroman]

Foundation added Locale.preferredLocales, but swift-dependencies currently only exposes @Dependency(\.locale).

The two values serve different purposes. locale is the app’s current formatting and regional locale. preferredLocales is an ordered list used for content-language selection and matching.

Would you be open to adding this?

@Dependency(\.preferredLocales) var preferredLocales

A possible implementation:

private enum PreferredLocalesKey: DependencyKey {
  static let liveValue: @Sendable () -> [Locale] = {
    Locale.preferredLanguages.compactMap {
      Locale(identifier: $0)
    }
  }

  static let testValue: @Sendable () -> [Locale] = unimplemented(
    #"@Dependency(\.preferredLocales)"#,
    placeholder: []
  )
}

extension DependencyValues {
  public var preferredLocales: [Locale] {
    get { self[PreferredLocalesKey.self]() }
    set { self[PreferredLocalesKey.self] = { newValue } }
  }
}

Tests could override it normally:

withDependencies {
  $0.preferredLocales = [Locale(identifier: "es-ES")]
} operation: {
  // ...
}

Swift Foundation implements Locale.preferredLocales in terms of Locale.preferredLanguages:

public static var preferredLocales: [Locale] {
  preferredLanguages.compactMap {
    Locale(identifier: $0)
  }
}

The implementation has no deployment constraints itself, so the dependency could be back-deployed rather than restricted to OS 26.

I’m happy to open a PR if this sounds useful.

[stephencelis]

This all sounds reasonable enough! Wanna open a PR?

[davdroman]

@stephencelis PR open in #468.