diff --git a/shell/platform/android/io/flutter/plugin/editing/SpellCheckPlugin.java b/shell/platform/android/io/flutter/plugin/editing/SpellCheckPlugin.java
index 1bfc503a73c4c..f74076c7af45b 100644
--- a/shell/platform/android/io/flutter/plugin/editing/SpellCheckPlugin.java
+++ b/shell/platform/android/io/flutter/plugin/editing/SpellCheckPlugin.java
@@ -89,7 +89,6 @@ public void initiateSpellCheck(
 
   /** Calls on the Android spell check API to spell check specified text. */
   public void performSpellCheck(@NonNull String locale, @NonNull String text) {
-    String[] localeCodes = locale.split("-");
     Locale localeFromString = LocalizationPlugin.localeFromString(locale);
 
     if (mSpellCheckerSession == null) {
diff --git a/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPlugin.mm b/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPlugin.mm
index 2eac491c3cefc..1b2038b5f63ea 100644
--- a/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPlugin.mm
+++ b/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPlugin.mm
@@ -61,6 +61,8 @@ - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
 // Returns an empty array if no spell check suggestions.
 - (NSArray<NSDictionary<NSString*, id>*>*)findAllSpellCheckSuggestionsForText:(NSString*)text
                                                                    inLanguage:(NSString*)language {
+  // Transform Dart Locale format to iOS language format.
+  language = [language stringByReplacingOccurrencesOfString:@"-" withString:@"_"];
   if (![UITextChecker.availableLanguages containsObject:language]) {
     return nil;
   }
@@ -145,7 +147,9 @@ - (instancetype)initWithMisspelledRange:(NSRange)range
 - (NSDictionary<NSString*, NSObject*>*)toDictionary {
   NSMutableDictionary* result = [[[NSMutableDictionary alloc] initWithCapacity:3] autorelease];
   result[@"startIndex"] = @(_misspelledRange.location);
-  result[@"endIndex"] = @(_misspelledRange.location + _misspelledRange.length - 1);
+  // The end index represents the next index after the last character of a misspelled word to match
+  // the behavior of Dart's TextRange: https://api.flutter.dev/flutter/dart-ui/TextRange/end.html
+  result[@"endIndex"] = @(_misspelledRange.location + _misspelledRange.length);
   result[@"suggestions"] = _suggestions;
   return result;
 }
diff --git a/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPluginTest.mm b/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPluginTest.mm
index 92fc4469b8872..221050638efea 100644
--- a/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPluginTest.mm
+++ b/shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPluginTest.mm
@@ -161,11 +161,11 @@ - (void)testFindAllSpellCheckSuggestionsForText {
   XCTAssertTrue(capturedResult.count == 2);
   NSDictionary* suggestionsJSON1 = capturedResult.firstObject;
   XCTAssertEqualObjects(suggestionsJSON1[@"startIndex"], @0);
-  XCTAssertEqualObjects(suggestionsJSON1[@"endIndex"], @4);
+  XCTAssertEqualObjects(suggestionsJSON1[@"endIndex"], @5);
   XCTAssertEqualObjects(suggestionsJSON1[@"suggestions"], suggestions1);
   NSDictionary* suggestionsJSON2 = capturedResult[1];
   XCTAssertEqualObjects(suggestionsJSON2[@"startIndex"], @5);
-  XCTAssertEqualObjects(suggestionsJSON2[@"endIndex"], @9);
+  XCTAssertEqualObjects(suggestionsJSON2[@"endIndex"], @10);
   XCTAssertEqualObjects(suggestionsJSON2[@"suggestions"], suggestions2);
   [self.mockTextChecker reset];
   [textCheckerClassMock stopMocking];
@@ -198,11 +198,11 @@ - (void)testStopFindingMoreWhenTheLastWordIsMisspelled {
   XCTAssertTrue(capturedResult.count == 2);
   NSDictionary* suggestionsJSON1 = capturedResult.firstObject;
   XCTAssertEqualObjects(suggestionsJSON1[@"startIndex"], @0);
-  XCTAssertEqualObjects(suggestionsJSON1[@"endIndex"], @4);
+  XCTAssertEqualObjects(suggestionsJSON1[@"endIndex"], @5);
   XCTAssertEqualObjects(suggestionsJSON1[@"suggestions"], suggestions1);
   NSDictionary* suggestionsJSON2 = capturedResult[1];
   XCTAssertEqualObjects(suggestionsJSON2[@"startIndex"], @6);
-  XCTAssertEqualObjects(suggestionsJSON2[@"endIndex"], @9);
+  XCTAssertEqualObjects(suggestionsJSON2[@"endIndex"], @10);
   XCTAssertEqualObjects(suggestionsJSON2[@"suggestions"], suggestions2);
   [self.mockTextChecker reset];
   [textCheckerClassMock stopMocking];
@@ -228,7 +228,7 @@ - (void)testStopFindingMoreWhenTheWholeStringIsAMisspelledWord {
   XCTAssertTrue(capturedResult.count == 1);
   NSDictionary* suggestionsJSON1 = capturedResult.firstObject;
   XCTAssertEqualObjects(suggestionsJSON1[@"startIndex"], @0);
-  XCTAssertEqualObjects(suggestionsJSON1[@"endIndex"], @4);
+  XCTAssertEqualObjects(suggestionsJSON1[@"endIndex"], @5);
   XCTAssertEqualObjects(suggestionsJSON1[@"suggestions"], suggestions1);
   [self.mockTextChecker reset];
   [textCheckerClassMock stopMocking];
@@ -270,6 +270,29 @@ - (void)testUnsupportedLanguageShouldReturnNil {
   [textCheckerClassMock stopMocking];
 }
 
+- (void)testSupportSubLanguage {
+  self.partialMockPlugin = OCMPartialMock(self.plugin);
+  OCMStub([self.partialMockPlugin textChecker]).andReturn(self.mockTextChecker);
+  id textCheckerClassMock = OCMClassMock([UITextChecker class]);
+  [[[textCheckerClassMock stub] andReturn:@[ @"en_us" ]] availableLanguages];
+  NSArray* suggestions1 = @[ @"suggestion 1", @"suggestion 2" ];
+
+  [self mockUITextCheckerWithExpectedMisspelledWordRange:NSMakeRange(0, 5)
+                                           startingIndex:0
+                                             suggestions:suggestions1];
+  __block NSArray* capturedResult;
+  [self.mockMethodChannel invokeMethod:@"SpellCheck.initiateSpellCheck"
+                             arguments:@[ @"en-us", @"hejjo" ]
+                                result:^(id _Nullable result) {
+                                  capturedResult = result;
+                                }];
+  NSDictionary* suggestionsJSON1 = capturedResult.firstObject;
+  XCTAssertEqualObjects(suggestionsJSON1[@"startIndex"], @0);
+  XCTAssertEqualObjects(suggestionsJSON1[@"endIndex"], @5);
+  XCTAssertEqualObjects(suggestionsJSON1[@"suggestions"], suggestions1);
+  [textCheckerClassMock stopMocking];
+}
+
 - (void)testEmptyStringShouldReturnEmptyResults {
   self.partialMockPlugin = OCMPartialMock(self.plugin);
   OCMStub([self.partialMockPlugin textChecker]).andReturn(self.mockTextChecker);
diff --git a/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm b/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm
index 8719855a7e5b8..d4ce938899df9 100644
--- a/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm
+++ b/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm
@@ -1654,7 +1654,8 @@ - (void)onUserSettingsChanged:(NSNotification*)notification {
     @"textScaleFactor" : @([self textScaleFactor]),
     @"alwaysUse24HourFormat" : @([self isAlwaysUse24HourFormat]),
     @"platformBrightness" : [self brightnessMode],
-    @"platformContrast" : [self contrastMode]
+    @"platformContrast" : [self contrastMode],
+    @"nativeSpellCheckServiceDefined" : @true
   }];
 }