Setting "AppleLanguages" doesn't change app language

Question

I am trying to implement a function that can change app language.
I tried to set it like this:

let defaults = NSUserDefaults.standardUserDefaults()
defaults.setObject("de", forKey: "AppleLanguages")

And after app restart language is still 'system default'.
Now I test different languages by setting Scheme > Application Language > language.
But is it possible to user click a button and after restart to see picked language?

Also what is the proper way to do this to avoid changing language on restart?

Answer 1

This solution work for me:
1. Create an extension of Bundle

    import Foundation

    class L012Localizer: NSObject {
        class func DoTheSwizzling() {
            MethodSwizzleGivenClassName(cls: Bundle.self, originalSelector: #selector(Bundle.localizedString(forKey:value:table:)), overrideSelector:
                #selector(Bundle.specialLocalizedString(key:value:table:)))
        }
    }

    extension Bundle {
        @objc func specialLocalizedString(key: String, value: String?, table tableName: String?) -> String {
            let currentLanguage = Utils.currentLanguage().rawValue
            var bundle = Bundle();
            if currentLanguage != "" , let _path = Bundle.main.path(forResource: currentLanguage, ofType: "lproj") {
                bundle = Bundle(path: _path)!
            } else {
                let _path = Bundle.main.path(forResource: "Base", ofType: "lproj")!
                bundle = Bundle(path: _path)!
            }
            return (bundle.specialLocalizedString(key: key, value: value, table: tableName))
        }
    }

    func MethodSwizzleGivenClassName(cls: AnyClass, originalSelector: Selector, overrideSelector: Selector){

        let origMethod: Method = class_getInstanceMethod(cls, originalSelector)!;
        let overrideMethod: Method = class_getInstanceMethod(cls, overrideSelector)!;
        if (class_addMethod(cls, originalSelector, method_getImplementation(overrideMethod), method_getTypeEncoding(overrideMethod))) {
            class_replaceMethod(cls, overrideSelector, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
        } else {
            method_exchangeImplementations(origMethod, overrideMethod);
        }
    }

2. Call DoTheSwizzling in AppDelegate

     func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
            L012Localizer.DoTheSwizzling()
            return true
        }

3. Create Language Utils

    class Utils: NSObject {
        class func setLanguage(_ lang: LanguageType) {
            UserDefaults.standard.set([lang.rawValue], forKey: "AppleLanguages")
        }

        class func currentLanguage() -> LanguageType {
            if let langs = UserDefaults.standard.object(forKey: "AppleLanguages") as? [String], let currentLang = langs.first {
                return LanguageType(rawValue: currentLang) ?? .english
            }

            return .english
        }
    }

4. Create Language Type

    enum LanguageType: String {
        case english = "en"
        case korea = "ko"
        case vietnamese = "vi-VN"

        func toString() -> String {
            switch self {
            case .korea:
                return "Korea".localized
            case .vietnamese:
                return "Vietnamese".localized
            default:
                return "English".localized
            }
        }
    }

5. Remember that you have to config Application Language in Scheme to SystemLanguage

6. Then Every time you need to localize your app, you only need to call. Utils.setLanguage({LanguageType})