Default locale to en

lib/renderer/extensions/i18n.js

@@ -19,10 +19,11 @@ const getExtensionMetadata = (extensionId) => {
 
 const getMessagesPath = (extensionId, language) => {
   const {srcDirectory, default_locale} = getExtensionMetadata(extensionId)
+  const defaultLocale = default_locale != null ? default_locale : 'en'
   const localesDirectory = path.join(srcDirectory, '_locales')
   let messagesPath = path.join(localesDirectory, language, 'messages.json')
   if (!fs.statSyncNoException(messagesPath)) {
-    messagesPath = path.join(localesDirectory, default_locale, 'messages.json')
+    messagesPath = path.join(localesDirectory, defaultLocale, 'messages.json')
   }
   return messagesPath
 }