feat: respect language preferences on Win/macOS

This commit partially fixes electron#18829

Chromium uses navigator.languages (Accept-Language) to determine
fallback fonts on Windows, especially kanji/han characters in CJK.

Previously the full preferences set to OS was not given to Chromium.
For instance, when user sets 'en-US, ja-JP' to Accept-Language,
while Chromium chooses Japanese font for kanji text, but Electron
chooses Chinese font.  This is because only the first language is given
to Accept-Language on Electron.

This patch is based on electron#15532

Co-authored-by: Nitish Sakhawalkar <nitsakh@icloud.com>
Co-authored-by: Kasumi Hanazuki <kasumi@rollingapple.net>

filenames.gni

@@ -545,6 +545,10 @@ filenames = {
     "shell/common/key_weak_map.h",
     "shell/common/keyboard_util.cc",
     "shell/common/keyboard_util.h",
+    "shell/common/language_util.h",
+    "shell/common/language_util_linux.cc",
+    "shell/common/language_util_mac.mm",
+    "shell/common/language_util_win.cc",
     "shell/common/mac/main_application_bundle.h",
     "shell/common/mac/main_application_bundle.mm",
     "shell/common/mouse_util.cc",

shell/browser/api/electron_api_web_contents.cc

@@ -89,6 +89,7 @@
 #include "shell/common/gin_converters/value_converter.h"
 #include "shell/common/gin_helper/dictionary.h"
 #include "shell/common/gin_helper/object_template_builder.h"
+#include "shell/common/language_util.h"
 #include "shell/common/mouse_util.h"
 #include "shell/common/node_includes.h"
 #include "shell/common/options_switches.h"
@@ -100,6 +101,7 @@
 #include "third_party/blink/public/mojom/frame/find_in_page.mojom.h"
 #include "third_party/blink/public/mojom/frame/fullscreen.mojom.h"
 #include "third_party/blink/public/mojom/messaging/transferable_message.mojom.h"
+#include "third_party/blink/public/mojom/renderer_preferences.mojom.h"
 #include "ui/base/cursor/cursor.h"
 #include "ui/base/mojom/cursor_type.mojom-shared.h"
 #include "ui/display/screen.h"
@@ -121,7 +123,6 @@
 #endif
 
 #if defined(OS_LINUX) || defined(OS_WIN)
-#include "third_party/blink/public/mojom/renderer_preferences.mojom.h"
 #include "ui/gfx/font_render_params.h"
 #endif
 
@@ -535,7 +536,22 @@ void WebContents::InitWithSessionAndOptions(
   managed_web_contents()->GetView()->SetDelegate(this);
 
   auto* prefs = web_contents()->GetMutableRendererPrefs();
-  prefs->accept_languages = g_browser_process->GetApplicationLocale();
+
+  // Collect preferred languages from OS and browser process. accept_languages
+  // affects HTTP header, navigator.languages, and CJK fallback font seleciton.
+  //
+  // Note that an application locale set to the browser process might be
+  // different with the one set to the preference list.
+  // (e.g. overridden with --lang)
+  std::string accept_languages =
+      g_browser_process->GetApplicationLocale() + ",";
+  for (auto const& language : electron::GetPreferredLanguages()) {
+    if (language == g_browser_process->GetApplicationLocale())
+      continue;
+    accept_languages += language + ",";
+  }
+  accept_languages.pop_back();
+  prefs->accept_languages = accept_languages;
 
 #if defined(OS_LINUX) || defined(OS_WIN)
   // Update font settings.

shell/common/language_util.h

@@ -0,0 +1,26 @@
+// Copyright (c) 2020 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef SHELL_COMMON_LANGUAGE_UTIL_H_
+#define SHELL_COMMON_LANGUAGE_UTIL_H_
+
+#include <string>
+#include <vector>
+
+#include "base/strings/string16.h"
+
+namespace electron {
+
+// Return a list of user preferred languages from OS. The list doesn't include
+// overrides from command line arguments.
+std::vector<std::string> GetPreferredLanguages();
+
+#if defined(OS_WIN)
+bool GetPreferredLanguagesUsingGlobalization(
+    std::vector<base::string16>* languages);
+#endif
+
+}  // namespace electron
+
+#endif  // SHELL_COMMON_LANGUAGE_UTIL_H_

shell/common/language_util_linux.cc

@@ -0,0 +1,16 @@
+// Copyright (c) 2020 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#include "shell/common/language_util.h"
+
+#include "ui/base/l10n/l10n_util.h"
+
+namespace electron {
+
+std::vector<std::string> GetPreferredLanguages() {
+  std::vector<std::string> languages = {l10n_util::GetApplicationLocale("")};
+  return languages;
+}
+
+}  // namespace electron

shell/common/language_util_mac.mm

@@ -0,0 +1,25 @@
+// Copyright (c) 2020 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#include "shell/common/language_util.h"
+
+#import <Cocoa/Cocoa.h>
+#include <string>
+#include <vector>
+
+#include "base/strings/sys_string_conversions.h"
+
+namespace electron {
+
+std::vector<std::string> GetPreferredLanguages() {
+  std::vector<std::string> languages;
+  [[NSLocale preferredLanguages]
+      enumerateObjectsUsingBlock:^(NSString* language, NSUInteger i,
+                                   BOOL* stop) {
+        languages.push_back(base::SysNSStringToUTF8(language));
+      }];
+  return languages;
+}
+
+}  // namespace electron

shell/common/language_util_win.cc

@@ -0,0 +1,76 @@
+// Copyright (c) 2020 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#include "shell/common/language_util.h"
+
+#include <roapi.h>
+#include <windows.system.userprofile.h>
+#include <wrl.h>
+
+#include "base/strings/sys_string_conversions.h"
+#include "base/win/core_winrt_util.h"
+#include "base/win/i18n.h"
+#include "base/win/win_util.h"
+#include "base/win/windows_version.h"
+
+namespace electron {
+
+std::vector<std::string> GetPreferredLanguages() {
+  std::vector<base::string16> languages16;
+
+  // Attempt to use API available on Windows 10 or later, which
+  // returns the full list of language preferences.
+  if (!GetPreferredLanguagesUsingGlobalization(&languages16)) {
+    base::win::i18n::GetThreadPreferredUILanguageList(&languages16);
+  }
+
+  std::vector<std::string> languages;
+  for (const auto& language : languages16) {
+    languages.push_back(base::SysWideToUTF8(language));
+  }
+  return languages;
+}
+
+bool GetPreferredLanguagesUsingGlobalization(
+    std::vector<base::string16>* languages) {
+  if (base::win::GetVersion() < base::win::Version::WIN10)
+    return false;
+  if (!base::win::ResolveCoreWinRTDelayload() ||
+      !base::win::ScopedHString::ResolveCoreWinRTStringDelayload())
+    return false;
+
+  base::win::ScopedHString guid = base::win::ScopedHString::Create(
+      RuntimeClass_Windows_System_UserProfile_GlobalizationPreferences);
+  Microsoft::WRL::ComPtr<
+      ABI::Windows::System::UserProfile::IGlobalizationPreferencesStatics>
+      prefs;
+
+  HRESULT hr =
+      base::win::RoGetActivationFactory(guid.get(), IID_PPV_ARGS(&prefs));
+  if (FAILED(hr))
+    return false;
+
+  ABI::Windows::Foundation::Collections::IVectorView<HSTRING>* langs;
+  hr = prefs->get_Languages(&langs);
+  if (FAILED(hr))
+    return false;
+
+  unsigned size;
+  hr = langs->get_Size(&size);
+  if (FAILED(hr))
+    return false;
+
+  for (unsigned i = 0; i < size; ++i) {
+    HSTRING hstr;
+    hr = langs->GetAt(i, &hstr);
+    if (SUCCEEDED(hr)) {
+      base::WStringPiece str = base::win::ScopedHString(hstr).Get();
+      languages->emplace_back(str.data(), str.size());
+    }
+  }
+
+  return true;
+}
+
+}  // namespace electron

spec-main/chromium-spec.ts

@@ -319,7 +319,8 @@ describe('chromium features', () => {
       const w = new BrowserWindow({ show: false });
       await w.loadURL('about:blank');
       const languages = await w.webContents.executeJavaScript('navigator.languages');
-      expect(languages).to.deep.equal([appLocale]);
+      expect(languages.length).to.be.greaterThan(0);
+      expect(languages).to.contain(appLocale);
     });
   });