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

chrome/common/extensions/extension.cc

Index: chrome/common/extensions/extension.cc
===================================================================
--- chrome/common/extensions/extension.cc	(revision 32098)
+++ chrome/common/extensions/extension.cc	(working copy)
@@ -1103,6 +1103,14 @@
     }
   }
 
+  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;