Add setLanguage to iOS plugin

* Add setLanguage method to iOS plugin in OneSignalPlugin.m

Author: tanaynigam

ios/Classes/OneSignalPlugin.m

@@ -142,6 +142,8 @@ - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
         [self setExternalUserId:call withResult:result];
     else if ([@"OneSignal#removeExternalUserId" isEqualToString:call.method])
         [self removeExternalUserId:call withResult:result];
+    else if ([@"OneSignal#setLanguage" isEqualToString:call.method])
+        [self setLanguage:call withResult:result];
     else if ([@"OneSignal#initNotificationOpenedHandlerParams" isEqualToString:call.method])
         [self initNotificationOpenedHandlerParams];
     else if ([@"OneSignal#initInAppMessageClickedHandlerParams" isEqualToString:call.method])
@@ -320,6 +322,14 @@ - (void)removeExternalUserId:(FlutterMethodCall *)call withResult:(FlutterResult
     }];
 }
 
+- (void)setLanguage:(FlutterMethodCall *)call withResult:(FlutterResult)result {
+    id language = call.arguments[@"language"];
+    if (language == [NSNull null])
+        language = nil;
+
+    [OneSignal setLanguage:language];
+}
+
 - (void)initNotificationOpenedHandlerParams {
     [OneSignal setNotificationOpenedHandler:^(OSNotificationOpenedResult * _Nonnull result) {
         [OneSignalPlugin.sharedInstance handleNotificationOpened:result];