Parse messages.json in ExtensionUnpacker (like we do for manifest)....

chrome/common/extensions/extension.h

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_COMMON_EXTENSIONS_EXTENSION_H_
 #define CHROME_COMMON_EXTENSIONS_EXTENSION_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 256 matching lines @@
   }
 
   // Getter/setter for l10n message bundle.
   const ExtensionMessageBundle* message_bundle() const {
     return message_bundle_.get();
   }
   void set_message_bundle(ExtensionMessageBundle* message_bundle) {
     message_bundle_.reset(message_bundle);
   }
 
+  const std::string default_locale() const { return default_locale_; }
+
   // Chrome URL overrides (see ExtensionOverrideUI).
   const URLOverrideMap& GetChromeURLOverrides() const {
     return chrome_url_overrides_;
   }
 
   // Runtime data:
   // Put dynamic data about the state of a running extension below.
 
   // Whether the background page, if any, is ready. We don't load other
   // components until then. If there is no background page, we consider it to
@@ skipping 112 matching lines @@
 
   // URL for fetching an update manifest
   GURL update_url_;
 
   // A copy of the manifest that this extension was created from.
   scoped_ptr<DictionaryValue> manifest_value_;
 
   // Handles the l10n messages replacement and parsing.
   scoped_ptr<ExtensionMessageBundle> message_bundle_;
 
+  // Default locale for fall back. Can be empty if extension is not localized.
+  std::string default_locale_;
+
   // A map of chrome:// hostnames (newtab, downloads, etc.) to Extension URLs
   // which override the handling of those URLs.
   URLOverrideMap chrome_url_overrides_;
 
   // Runtime data:
 
   // True if the background page is ready.
   bool background_page_ready_;
 
   FRIEND_TEST(ExtensionTest, LoadPageActionHelper);
 
   DISALLOW_COPY_AND_ASSIGN(Extension);
 };
 
 #endif  // CHROME_COMMON_EXTENSIONS_EXTENSION_H_