Bug 1678712 - Support AXLangauge. r=morgan

The important part here is to have the text leafs inherit their parents' language.

Differential Revision: https://phabricator.services.mozilla.com/D212604

accessible/mac/MOXAccessibleProtocol.h

@@ -338,6 +338,9 @@
 // AXPlaceholderValue
 - (NSString* _Nullable)moxPlaceholderValue;
 
+// AXLanguage
+- (NSString* _Nullable)moxLanguage;
+
 // AXMozDebugDescription
 - (NSString* _Nullable)moxMozDebugDescription;
 

accessible/mac/mozAccessible.h

@@ -245,6 +245,9 @@ enum CheckedState {
 // override
 - (id)moxFocusableAncestor;
 
+// override
+- (NSString*)moxLanguage;
+
 #ifndef RELEASE_OR_BETA
 // override
 - (NSString*)moxMozDebugDescription;

accessible/mac/mozAccessible.mm

@@ -757,6 +757,15 @@ struct RoleDescrComparator {
   return [self moxEditableAncestor];
 }
 
+- (NSString*)moxLanguage {
+  MOZ_ASSERT(mGeckoAccessible);
+
+  nsAutoString lang;
+  mGeckoAccessible->Language(lang);
+
+  return nsCocoaUtils::ToNSString(lang);
+}
+
 #ifndef RELEASE_OR_BETA
 - (NSString*)moxMozDebugDescription {
   NS_OBJC_BEGIN_TRY_BLOCK_RETURN;