[iOS] Change locale format for spell check (#38080)

* Change format

* Add pointer

* Check length

* Add check

Author: camsim99

shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPlugin.mm

@@ -61,8 +61,14 @@ - (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:@"_"];
+  // Transform Dart Locale format to iOS language format if necessary.
+  if ([language containsString:@"-"]) {
+    NSArray<NSString*>* languageCodes = [language componentsSeparatedByString:@"-"];
+    FML_DCHECK(languageCodes.count == 2);
+    NSString* lastCode = [[languageCodes lastObject] uppercaseString];
+    language = [NSString stringWithFormat:@"%@_%@", [languageCodes firstObject], lastCode];
+  }
+
   if (![UITextChecker.availableLanguages containsObject:language]) {
     return nil;
   }

shell/platform/darwin/ios/framework/Source/FlutterSpellCheckPluginTest.mm

@@ -274,7 +274,7 @@ - (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];
+  [[[textCheckerClassMock stub] andReturn:@[ @"en_US" ]] availableLanguages];
   NSArray* suggestions1 = @[ @"suggestion 1", @"suggestion 2" ];
 
   [self mockUITextCheckerWithExpectedMisspelledWordRange:NSMakeRange(0, 5)