Force NSLocalizedString to use a specific language using Swift

@Radu I also made this working for XCUITests thanks to @Markus' original answer :

You can specify explicitly the path to your MainBundle, it will only work on your Mac with the Simulator, but it is often used in continuous integration platforms so this might be acceptable :

let language: String = "en"
let path = "/Users/{username}/{path_to_your_project}/\(language).lproj"
let bundle = Bundle(path: path)
let string = bundle?.localizedString(forKey: "key", value: nil, table: nil)

With NSLocalizedString you can specify the bundle.

let language = "fr"
let path = Bundle.main.path(forResource: language, ofType: "lproj")!
let bundle = Bundle(path: path)!
let localizedString = NSLocalizedString(key, bundle: bundle, comment: "")

Or with a bundle, you may also call localizedStringForKey:value:table: directly too.

It's not possible to change app's language immediately by changing the value of AppleLanguages. It requires restarting the app before the change takes effect.

It seems that your problem is accessing the localization strings of different languages rather than changing the app's language, right? If you want your app to support multiple languages, you can just provide the translations and rely on settings.app for the actual change.

If you want to access the localization strings from other than currently used localization, you need to get access to the proper translations bundle. And then just query that bundle for the translations. The following piece of code should do the trick.

let language = "en"
let path = Bundle.main.path(forResource: language, ofType: "lproj")
let bundle = Bundle(path: path!)
let string = bundle?.localizedStringForKey("key", value: nil, table: nil)