Update the native UI language without restarting the app

[1280103995]

It is better to not restart the app when changing the language in the app.

I asked AI to refer to your code and update the native UI on Android without restarting the app, but I couldn't solve it on iOS. I found this library, which claims to support iOS to switch the in-app language without restarting the app.

I have verified it on the library @react-native-community/datetimepicker: 8.3.0, and it works fine.

Here is the code for Android:

public class LocalizeModule extends ReactContextBaseJavaModule {

    private ReactApplicationContext reactContext;
    private static final String TAG = "LocalizeModule";

    public LocalizeModule(ReactApplicationContext reactContext) {
        super(reactContext);
        this.reactContext = reactContext;
    }

    @Override
    public String getName() {
        return "LocalizeModule";
    }

    /**
     * Get IETF BCP 47 (language-COUNTRY "pl-PL")
     * if country is not available in locale, then use system defaults (even if it's not 100% correct, like "pl-US")
     **/
    private String getLanguageTag(String language) {
        Locale locale = Locale.forLanguageTag(language);
        if (!TextUtils.isEmpty(locale.getCountry())) {
            return locale.toLanguageTag();
        }
        return new Locale(locale.getLanguage(), Locale.getDefault().getCountry()).toLanguageTag();
    }
    
    @ReactMethod
    public void getLanguage(Promise promise) {
        try {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
                String currentLocaleName;
                if (!AppCompatDelegate.getApplicationLocales().isEmpty()) {
                    currentLocaleName = AppCompatDelegate.getApplicationLocales().get(0).toLanguageTag();
                } else {
                    currentLocaleName = Locale.getDefault().toLanguageTag();
                }
                promise.resolve(currentLocaleName);
            } else {
                SharedPreferences prefs = getPreferences();
                String savedLanguageFrom = prefs.getString("languageFrom", null);
                
                if (savedLanguageFrom != null && savedLanguageFrom.equals(Locale.getDefault().getLanguage())) {
                    promise.resolve(prefs.getString("language", Locale.getDefault().toLanguageTag()));
                } else {
                    promise.resolve(Locale.getDefault().toLanguageTag());
                }
            }
        } catch (Exception e) {
            Log.e(TAG, "Error getting language", e);
            promise.reject("ERROR", "Failed to get language: " + e.getMessage());
        }
    }

    /**
     * passed language can be in ISO 639-1 (language "pl")
     * or IETF BCP 47 (language-COUNTRY "pl-PL")
     *  If API version >= 33, use native per-app language feature
     *  else, fallback to SharedPreferences
     **/
    @ReactMethod
    public void changeLanguage(String language, Promise promise) {
        try {
            Log.d(TAG, "Setting language to: " + language);

            String languageTag = getLanguageTag(language);
            Locale locale = Locale.forLanguageTag(languageTag);

            SharedPreferences.Editor editor = getPreferences().edit();
            editor.putString("languageFrom", Locale.getDefault().getLanguage());
            editor.putString("language", languageTag);
            editor.apply();
            
            Locale.setDefault(locale);
            
            Activity currentActivity = getCurrentActivity();
            if (currentActivity != null) {
                currentActivity.runOnUiThread(() -> {
                    try {
                        Configuration config = new Configuration(currentActivity.getResources().getConfiguration());
                        
                        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                            LocaleList localeList = new LocaleList(locale);
                            config.setLocales(localeList);
                        } else {
                            config.locale = locale;
                        }
                        
                        currentActivity.getResources().updateConfiguration(config, currentActivity.getResources().getDisplayMetrics());
                        
                        Context appContext = reactContext.getApplicationContext();
                        appContext.getResources().updateConfiguration(config, appContext.getResources().getDisplayMetrics());
                        
                        View rootView = currentActivity.getWindow().getDecorView().findViewById(android.R.id.content);
                        if (rootView != null) {
                            rootView.invalidate();
                        }
                        
                        Log.d(TAG, "Activity resources updated");
                    } catch (Exception e) {
                        Log.e(TAG, "Error updating activity resources", e);
                    }
                });
            }
            
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
                LocaleListCompat localeList = LocaleListCompat.forLanguageTags(languageTag);
                AppCompatDelegate.setApplicationLocales(localeList);
            }
            
            promise.resolve(true);
            
            Log.d(TAG, "Language set successfully to: " + language);
        } catch (Exception e) {
            Log.e(TAG, "Error setting language", e);
            promise.reject("ERROR", "Failed to set language: " + e.getMessage());
        }
    }

    /**
     * SharedPreferences (only used when API version is below 33)
     **/
    private SharedPreferences getPreferences() {
        return reactContext.getSharedPreferences(
            "LocalizationSettings", Context.MODE_PRIVATE
        );
    }
}