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

chrome/common/extensions/extension.cc

 // 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.
 
 #include "chrome/common/extensions/extension.h"
 
 #include "app/resource_bundle.h"
 #include "base/basictypes.h"
 #include "base/command_line.h"
 #include "base/file_path.h"
@@ skipping 1085 matching lines @@
       }
 
       // The path component is not used for host permissions, so we force it to
       // match all paths.
       pattern.set_path("/*");
 
       host_permissions_.push_back(pattern);
     }
   }
 
+  if (source.HasKey(keys::kDefaultLocale)) {
+    if (!source.GetString(keys::kDefaultLocale, &default_locale_) ||
+        default_locale_.empty()) {
+      *error = errors::kInvalidDefaultLocale;
+      return false;
+    }
+  }
+
   // Chrome URL overrides (optional)
   if (source.HasKey(keys::kChromeURLOverrides)) {
     DictionaryValue* overrides;
     if (!source.GetDictionary(keys::kChromeURLOverrides, &overrides)) {
       *error = errors::kInvalidChromeURLOverrides;
       return false;
     }
 
     // Validate that the overrides are all strings
     DictionaryValue::key_iterator iter = overrides->begin_keys();
@@ skipping 129 matching lines @@
     UserScript::PatternList::const_iterator pattern =
         content_script->url_patterns().begin();
     for (; pattern != content_script->url_patterns().end(); ++pattern) {
       if (pattern->match_subdomains() && pattern->host().empty())
         return true;
     }
   }
 
   return false;
 }