Add 'lang' to HTML tag

src/invidious/views/embed.ecr

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html>
+<html lang="<%= env.get("preferences").as(Preferences).locale %>">
 
 <head>
   <meta charset="utf-8">

src/invidious/views/template.ecr

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html>
+<html lang="<%= env.get("preferences").as(Preferences).locale %>">
 
 <head>
     <meta charset="utf-8">