Use default locale for html lang in layouts (#2212)

app/views/layouts/application.html.haml

 !!! 5
-%html{:lang => 'en'}
+%html{:lang => I18n.locale}
   %head
     %meta{:charset => 'utf-8'}/
     %meta{:name => 'viewport', :content => 'width=device-width, initial-scale=1'}/

app/views/layouts/embedded.html.haml

 !!! 5
-%html{:lang => 'en'}
+%html{:lang => I18n.locale}
   %head
     %meta{:charset => 'utf-8'}/
     = stylesheet_link_tag 'application', media: 'all'

app/views/layouts/error.html.haml

 !!!
-%html{:lang => "en"}
+%html{:lang => I18n.locale}
   %head
     %meta{:content => "text/html; charset=UTF-8", "http-equiv" => "Content-Type"}/
     %meta{:charset => "utf-8"}/