fix(a11y): html lang should reflect locale

fix #7150

app/helpers/application_helper.rb

@@ -1,6 +1,10 @@
 module ApplicationHelper
   include SanitizeUrl
 
+  def html_lang
+    I18n.locale.to_s
+  end
+
   def sanitize_url(url)
     if !url.nil?
       super(url, schemes: ['http', 'https'], replace_evil_with: root_url)

app/views/layouts/application.html.haml

@@ -1,5 +1,5 @@
 !!! 5
-%html{ lang: "fr", class: yield(:root_class) }
+%html{ lang: html_lang, class: yield(:root_class) }
   %head
     %meta{ "http-equiv": "Content-Type", content: "text/html; charset=UTF-8" }
     %meta{ "http-equiv": "X-UA-Compatible", content: "IE=edge" }