An implementation of a LocalizableString type in the Swift new-interpolation branch: https://github.com/brentdax/swift/tree/new-interpolation

LocalizableString-new-interpolation.swift

import Foundation

public protocol LocalizableArgument {
  var localizableFormat: String { get }
  var localizableFormatArguments: [CVarArg] { get }
}

extension LocalizableArgument where Self: CVarArg {
  public var localizableFormatArguments: [CVarArg] {
    return [self]
  }
}

extension String: LocalizableArgument {
  public var localizableFormat: String {
    return "%@"
  }
  
  public var localizableFormatArgument: CVarArg {
    return self as NSString as CVarArg
  }
}

public struct LocalizableString: LocalizableArgument {
  public var localizableFormat: String
  public var localizableFormatArguments: [CVarArg]
  
  public func localizedFormat(inTable tableName: String? = nil, from bundle: Bundle = .main) -> String {
    return bundle.localizedString(forKey: localizableFormat, value: nil, table: tableName)
  }
  
  public init(localizableFormat: String, arguments: [CVarArg]) {
    self.localizableFormat = localizableFormat
    self.localizableFormatArguments = arguments
  }
  
  public init(escaping plainText: String) {
    let escapedScalars = plainText.unicodeScalars.flatMap { $0 == "%" ? [$0, $0] : [$0] }
    self.init(localizableFormat: String(String.UnicodeScalarView(escapedScalars)), arguments: [])
  }
  
  public init(_ argument: LocalizableArgument) {
    self.init(localizableFormat: argument.localizableFormat, arguments: argument.localizableFormatArguments)
  }
}

public func localized(_ string: LocalizableString, comment: String = "", inTable tableName: String? = nil, from bundle: Bundle = .main) -> String {
  let format = string.localizedFormat(inTable: tableName, from: bundle)
  return String(format: format, locale: .current, arguments: string.localizableFormatArguments)
}

public func += (lhs: inout LocalizableString, rhs: LocalizableArgument) {
  lhs.localizableFormat += rhs.localizableFormat
  lhs.localizableFormatArguments += rhs.localizableFormatArguments
}

public func + (lhs: LocalizableArgument, rhs: LocalizableArgument) -> LocalizableString {
  var copy = LocalizableString(lhs)
  copy += rhs
  return copy
}

extension LocalizableString: ExpressibleByStringInterpolation {
  public init(stringLiteral value: String) {
    self.init(escaping: value)
  }
  
  public init<T>(stringInterpolationSegment expr: T) {
    if let expr = expr as? LocalizableArgument {
      self.init(expr)
    }
    else {
      self.init(String(describing: expr))
    }
  }
  
  public init(stringInterpolation segments: LocalizableString...) {
    self.init(escaping: "")
    for segment in segments {
      self += segment
    }
  }
  
  public init(extendedGraphemeClusterLiteral value: String) {
    self.init(stringLiteral: value)
  }
  public init(unicodeScalarLiteral value: String) {
    self.init(extendedGraphemeClusterLiteral: value)
  }
}

extension Int: LocalizableArgument {
  public var localizableFormat: String { return "%d" }
}
extension Double: LocalizableArgument {
  public var localizableFormat: String { return "%f" }
}
extension NSObject: LocalizableArgument {
  public var localizableFormat: String { return "%@" }
}


print("Enter your name: ", terminator: "")
guard let name = readLine() else { print("Byeeeee"); exit(1) }

let s: LocalizableString = "Hello, \(name)!"
print(s.localizableFormat)
print(s.localizableFormatArguments)

print(localized("Hello, \(name)!"))