Change to more generic two letter code

Read more: https://jsonfeed.org/version/1.1

_data/metadata.json

@@ -1,7 +1,7 @@
 {
   "title": "Your Blog Name",
   "url": "https://example.com/",
-  "language": "en-US",
+  "language": "en",
   "description": "I am writing about my experiences as a naval navel-gazer.",
   "feed": {
     "subtitle": "I am writing about my experiences as a naval navel-gazer.",