add callbacks to setLanguage method

- Android: added OSFlutterSetLanguageHandler that implements the OSSetLanguageCompletionHandler

Author: nan-li

android/src/main/java/com/onesignal/flutter/OneSignalPlugin.java

@@ -329,7 +329,7 @@ private void setLanguage(MethodCall call, final Result result) {
     if (language != null && language.length() == 0)
       language = null;
 
-      OneSignal.setLanguage(language);
+      OneSignal.setLanguage(language, new OSFlutterSetLanguageHandler(messenger, channel, result, "setLanguage"));
   }
 
   private void setExternalUserId(MethodCall call, final Result result) {
@@ -603,6 +603,47 @@ public void onFailure(JSONObject response) {
     }
   }
 
+  static class OSFlutterSetLanguageHandler extends OSFlutterHandler
+          implements OneSignal.OSSetLanguageCompletionHandler {
+
+    OSFlutterSetLanguageHandler(BinaryMessenger messenger, MethodChannel channel, Result res, String methodName) {
+      super(messenger, channel, res, methodName);
+    }
+
+    @Override
+    public void onSuccess(String results) {
+      if (this.replySubmitted.getAndSet(true)) {
+        OneSignal.onesignalLog(OneSignal.LOG_LEVEL.DEBUG, "OneSignal " + methodName + " handler called twice, ignoring! response: " + results);
+        return;
+      }
+
+      if (results == null) { // The results currently is always null.
+        results = "Successfully set language.";
+      }
+
+      HashMap<String, Object> responseMap = new HashMap<>();
+      responseMap.put("success", true);
+      responseMap.put("message", results);
+      replySuccess(result, responseMap);
+    }
+
+    @Override
+    public void onFailure(OneSignal.OSLanguageError error) {
+      if (this.replySubmitted.getAndSet(true)) {
+        return;
+      }
+
+      String errorMessage = error.getMessage();
+      if (errorMessage == null) {
+        errorMessage = "Failed to set language.";
+      }
+
+      replyError(result, "OneSignal",
+              "Encountered an error when " + methodName + ": " + errorMessage,
+              null);
+    }
+  }
+
   static class OSFlutterHandler extends FlutterRegistrarResponder {
     protected final Result result;
     protected final String methodName;

ios/Classes/OneSignalPlugin.m

@@ -338,7 +338,12 @@ - (void)setLanguage:(FlutterMethodCall *)call withResult:(FlutterResult)result {
         language = nil;
     }
 
-    [OneSignal setLanguage:language];
+    [OneSignal setLanguage:language withSuccess:^(NSDictionary *results) {
+        result(results);
+    } withFailure:^(NSError *error) {
+        [OneSignal onesignalLog:ONE_S_LL_VERBOSE message:[NSString stringWithFormat:@"Set language Failure with error: %@", error]];
+        result(error.flutterError);
+    }];
 }
 
 - (void)initNotificationOpenedHandlerParams {