diff --git a/Source/WebKit/Platform/spi/Cocoa/FoundationSPI.h b/Source/WebKit/Platform/spi/Cocoa/FoundationSPI.h
index 916b765e5f23..3f19c074764a 100644
--- a/Source/WebKit/Platform/spi/Cocoa/FoundationSPI.h
+++ b/Source/WebKit/Platform/spi/Cocoa/FoundationSPI.h
@@ -28,6 +28,7 @@
 #if USE(APPLE_INTERNAL_SDK)
 
 #import <Foundation/NSExtension.h>
+#import <Foundation/NSLocale_Private.h>
 #import <Foundation/NSPrivateDecls.h>
 
 #if PLATFORM(IOS_FAMILY)
@@ -59,6 +60,10 @@ NS_ASSUME_NONNULL_BEGIN
 @property (copy, NS_NONATOMIC_IOSONLY) void (^requestInterruptionBlock)(id <NSCopying>);
 @end
 
+@interface NSLocale ()
++ (NSString *)_deviceLanguage;
+@end
+
 NS_ASSUME_NONNULL_END
 
 #endif // USE(APPLE_INTERNAL_SDK)
diff --git a/Source/WebKit/Shared/Extensions/WebExtensionUtilities.mm b/Source/WebKit/Shared/Extensions/WebExtensionUtilities.mm
index 6ae2b572d1f8..6cba68b6f9ec 100644
--- a/Source/WebKit/Shared/Extensions/WebExtensionUtilities.mm
+++ b/Source/WebKit/Shared/Extensions/WebExtensionUtilities.mm
@@ -368,8 +368,8 @@ JSObjectRef toJSRejectedPromise(JSContextRef context, NSString *callingAPIName,
 
 NSString *toWebAPI(NSLocale *locale)
 {
-    if (!locale.languageCode)
-        return nil;
+    if (!locale.languageCode.length)
+        return @"und";
 
     if (locale.countryCode.length)
         return [NSString stringWithFormat:@"%@-%@", locale.languageCode, locale.countryCode];
diff --git a/Source/WebKit/WebProcess/Extensions/API/Cocoa/WebExtensionAPILocalizationCocoa.mm b/Source/WebKit/WebProcess/Extensions/API/Cocoa/WebExtensionAPILocalizationCocoa.mm
index 5e0545e08f61..c12f8bf09a0a 100644
--- a/Source/WebKit/WebProcess/Extensions/API/Cocoa/WebExtensionAPILocalizationCocoa.mm
+++ b/Source/WebKit/WebProcess/Extensions/API/Cocoa/WebExtensionAPILocalizationCocoa.mm
@@ -31,6 +31,7 @@
 #import "WebExtensionAPILocalization.h"
 
 #import "CocoaHelpers.h"
+#import "FoundationSPI.h"
 #import "Logging.h"
 #import "MessageSenderInlines.h"
 #import "WebExtensionContextMessages.h"
@@ -88,6 +89,16 @@
     callback->call(acceptLanguages.array);
 }
 
+void WebExtensionAPILocalization::getPreferredSystemLanguages(Ref<WebExtensionCallbackHandler>&& callback)
+{
+    callback->call(NSLocale.preferredLanguages);
+}
+
+void WebExtensionAPILocalization::getSystemUILanguage(Ref<WebExtensionCallbackHandler>&& callback)
+{
+    callback->call(NSLocale._deviceLanguage);
+}
+
 } // namespace WebKit
 
 #endif // ENABLE(WK_WEB_EXTENSIONS)
diff --git a/Source/WebKit/WebProcess/Extensions/API/WebExtensionAPILocalization.h b/Source/WebKit/WebProcess/Extensions/API/WebExtensionAPILocalization.h
index 1d79ddbbc51c..a7b16ff1a395 100644
--- a/Source/WebKit/WebProcess/Extensions/API/WebExtensionAPILocalization.h
+++ b/Source/WebKit/WebProcess/Extensions/API/WebExtensionAPILocalization.h
@@ -39,6 +39,8 @@ class WebExtensionAPILocalization : public WebExtensionAPIObject, public JSWebEx
     NSString *getMessage(NSString* messageName, id substitutions);
     NSString *getUILanguage();
     void getAcceptLanguages(Ref<WebExtensionCallbackHandler>&&);
+    void getPreferredSystemLanguages(Ref<WebExtensionCallbackHandler>&&);
+    void getSystemUILanguage(Ref<WebExtensionCallbackHandler>&&);
 };
 
 } // namespace WebKit
diff --git a/Source/WebKit/WebProcess/Extensions/Interfaces/WebExtensionAPILocalization.idl b/Source/WebKit/WebProcess/Extensions/Interfaces/WebExtensionAPILocalization.idl
index 7751018aac90..30d012f23b7d 100644
--- a/Source/WebKit/WebProcess/Extensions/Interfaces/WebExtensionAPILocalization.idl
+++ b/Source/WebKit/WebProcess/Extensions/Interfaces/WebExtensionAPILocalization.idl
@@ -36,4 +36,7 @@
 
     void getAcceptLanguages([Optional, CallbackHandler] function callback);
 
+    void getPreferredSystemLanguages([Optional, CallbackHandler] function callback);
+
+    void getSystemUILanguage([Optional, CallbackHandler] function callback);
 };
diff --git a/Tools/TestWebKitAPI/Tests/WebKitCocoa/WKWebExtensionAPILocalization.mm b/Tools/TestWebKitAPI/Tests/WebKitCocoa/WKWebExtensionAPILocalization.mm
index 61dc6ce6f8be..1c9bb5025ad5 100644
--- a/Tools/TestWebKitAPI/Tests/WebKitCocoa/WKWebExtensionAPILocalization.mm
+++ b/Tools/TestWebKitAPI/Tests/WebKitCocoa/WKWebExtensionAPILocalization.mm
@@ -34,6 +34,10 @@
 #import "CocoaHelpers.h"
 #import "WebExtensionUtilities.h"
 
+@interface NSLocale ()
++ (NSString *)_deviceLanguage;
+@end
+
 static NSString * const messageKey = @"message";
 static NSString * const placeholdersKey = @"placeholders";
 static NSString * const placeholderDictionaryContentKey = @"content";
@@ -77,8 +81,8 @@
 
 static NSString *localeStringInWebExtensionFormat(NSLocale *locale)
 {
-    if (!locale.languageCode)
-        return @"";
+    if (!locale.languageCode.length)
+        return @"und";
 
     if (locale.countryCode.length)
         return [NSString stringWithFormat:@"%@-%@", locale.languageCode, locale.countryCode];
@@ -86,6 +90,7 @@
 }
 
 static NSString *currentLocaleString = localeStringInWebExtensionFormat(currentLocale);
+static NSString *currentSystemLocaleString = NSLocale._deviceLanguage;
 
 static auto *baseURLString = @"test-extension://76C788B8-3374-400D-8259-40E5B9DF79D3";
 
@@ -122,10 +127,13 @@
     }
 
     auto *acceptedLanguagesString = Util::constructJSArrayOfStrings(acceptedLanguages.array);
+    auto *preferredLanguagesString = Util::constructJSArrayOfStrings(preferredLocaleIdentifiers);
 
     auto *backgroundScript = Util::constructScript(@[
         [NSString stringWithFormat:@"const acceptedLanguages = %@", acceptedLanguagesString],
+        [NSString stringWithFormat:@"const preferredLanguages = %@", preferredLanguagesString],
         [NSString stringWithFormat:@"const currentUILanguage = '%@'", currentLocaleString],
+        [NSString stringWithFormat:@"const currentSystemUILanguage = '%@'", currentSystemLocaleString],
         [NSString stringWithFormat:@"const textDirection = '%@'", textDirection],
         [NSString stringWithFormat:@"const reversedTextDirection = '%@'", reversedTextDirection],
         [NSString stringWithFormat:@"const startEdge = '%@'", startEdge],
@@ -140,9 +148,14 @@
         @"browser.test.assertEq(browser.i18n.getMessage('@@bidi_end_edge'), endEdge)",
         @"browser.test.assertEq(browser.i18n.getMessage('unknown_message'), '')",
 
+        @"browser.test.assertFalse(currentUILanguage === 'und')",
         @"browser.test.assertEq(browser.i18n.getUILanguage(), currentUILanguage)",
         @"browser.test.assertDeepEq(await browser.i18n.getAcceptLanguages(), acceptedLanguages)",
 
+        @"browser.test.assertFalse(currentSystemUILanguage === 'und')",
+        @"browser.test.assertEq(await browser.i18n.getSystemUILanguage(), currentSystemUILanguage)",
+        @"browser.test.assertDeepEq(await browser.i18n.getPreferredSystemLanguages(), preferredLanguages)",
+
         @"browser.test.notifyPass()",
     ]);
 
@@ -166,10 +179,13 @@
     }
 
     auto *acceptedLanguagesString = Util::constructJSArrayOfStrings(acceptedLanguages.array);
+    auto *preferredLanguagesString = Util::constructJSArrayOfStrings(preferredLocaleIdentifiers);
 
     auto *backgroundScript = Util::constructScript(@[
         [NSString stringWithFormat:@"const acceptedLanguages = %@", acceptedLanguagesString],
+        [NSString stringWithFormat:@"const preferredLanguages = %@", preferredLanguagesString],
         [NSString stringWithFormat:@"const currentUILanguage = '%@'", currentLocaleString],
+        [NSString stringWithFormat:@"const currentSystemUILanguage = '%@'", currentSystemLocaleString],
         [NSString stringWithFormat:@"const textDirection = '%@'", textDirection],
         [NSString stringWithFormat:@"const reversedTextDirection = '%@'", reversedTextDirection],
         [NSString stringWithFormat:@"const startEdge = '%@'", startEdge],
@@ -186,9 +202,14 @@
         @"browser.test.assertEq(browser.i18n.getMessage('@@bidi_end_edge'), endEdge)",
         @"browser.test.assertEq(browser.i18n.getMessage('unknown_message'), '')",
 
+        @"browser.test.assertFalse(currentUILanguage === 'und')",
         @"browser.test.assertEq(browser.i18n.getUILanguage(), currentUILanguage)",
         @"browser.test.assertDeepEq(await browser.i18n.getAcceptLanguages(), acceptedLanguages)",
 
+        @"browser.test.assertFalse(currentSystemUILanguage === 'und')",
+        @"browser.test.assertEq(await browser.i18n.getSystemUILanguage(), currentSystemUILanguage)",
+        @"browser.test.assertDeepEq(await browser.i18n.getPreferredSystemLanguages(), preferredLanguages)",
+
         @"browser.test.notifyPass()",
     ]);
 
@@ -249,10 +270,13 @@
     }
 
     auto *acceptedLanguagesString = Util::constructJSArrayOfStrings(acceptedLanguages.array);
+    auto *preferredLanguagesString = Util::constructJSArrayOfStrings(preferredLocaleIdentifiers);
 
     auto *backgroundScript = Util::constructScript(@[
         [NSString stringWithFormat:@"const acceptedLanguages = %@", acceptedLanguagesString],
+        [NSString stringWithFormat:@"const preferredLanguages = %@", preferredLanguagesString],
         [NSString stringWithFormat:@"const currentUILanguage = '%@'", currentLocaleString],
+        [NSString stringWithFormat:@"const currentSystemUILanguage = '%@'", currentSystemLocaleString],
 
         @"browser.test.assertEq(browser.i18n.getMessage('@@extension_id'), '76C788B8-3374-400D-8259-40E5B9DF79D3')",
         @"browser.test.assertEq(browser.i18n.getMessage('@@ui_locale'), '')",
@@ -262,9 +286,14 @@
         @"browser.test.assertEq(browser.i18n.getMessage('@@bidi_end_edge'), '')",
         @"browser.test.assertEq(browser.i18n.getMessage('unknown_message'), '')",
 
+        @"browser.test.assertFalse(currentUILanguage === 'und')",
         @"browser.test.assertEq(browser.i18n.getUILanguage(), currentUILanguage)",
         @"browser.test.assertDeepEq(await browser.i18n.getAcceptLanguages(), acceptedLanguages)",
 
+        @"browser.test.assertFalse(currentSystemUILanguage === 'und')",
+        @"browser.test.assertEq(await browser.i18n.getSystemUILanguage(), currentSystemUILanguage)",
+        @"browser.test.assertDeepEq(await browser.i18n.getPreferredSystemLanguages(), preferredLanguages)",
+
         @"browser.test.notifyPass()",
     ]);
 
@@ -302,10 +331,13 @@
     };
 
     auto *acceptedLanguagesString = Util::constructJSArrayOfStrings(acceptedLanguages.array);
+    auto *preferredLanguagesString = Util::constructJSArrayOfStrings(preferredLocaleIdentifiers);
 
     auto *backgroundScript = Util::constructScript(@[
         [NSString stringWithFormat:@"const acceptedLanguages = %@", acceptedLanguagesString],
+        [NSString stringWithFormat:@"const preferredLanguages = %@", preferredLanguagesString],
         [NSString stringWithFormat:@"const currentUILanguage = '%@'", currentLocaleString],
+        [NSString stringWithFormat:@"const currentSystemUILanguage = '%@'", currentSystemLocaleString],
 
         @"browser.test.assertEq(browser.i18n.getMessage('@@extension_id'), '76C788B8-3374-400D-8259-40E5B9DF79D3')",
         @"browser.test.assertEq(browser.i18n.getMessage('@@ui_locale'), '')",
@@ -315,9 +347,14 @@
         @"browser.test.assertEq(browser.i18n.getMessage('@@bidi_end_edge'), '')",
         @"browser.test.assertEq(browser.i18n.getMessage('unknown_message'), '')",
 
+        @"browser.test.assertFalse(currentUILanguage === 'und')",
         @"browser.test.assertEq(browser.i18n.getUILanguage(), currentUILanguage)",
         @"browser.test.assertDeepEq(await browser.i18n.getAcceptLanguages(), acceptedLanguages)",
 
+        @"browser.test.assertFalse(currentSystemUILanguage === 'und')",
+        @"browser.test.assertEq(await browser.i18n.getSystemUILanguage(), currentSystemUILanguage)",
+        @"browser.test.assertDeepEq(await browser.i18n.getPreferredSystemLanguages(), preferredLanguages)",
+
         @"browser.test.notifyPass()",
     ]);