Create initial language support

Create support for languages, then reformat user, home, and post pages
to use it, and create en and en-us language files.

src/lib/constants.js

@@ -107,6 +107,7 @@ let constants = {
 	},
 
 	default_user_settings: {
+		language: "en",
 		rewrite_youtube: "invidio.us",
 		rewrite_twitter: "nitter.net"
 	},