HREFLANG for multi regional site

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Kenneth Villegas: Hi guys! Just want to ask regarding hreflang for multi regional site. I'm having trouble implementing on our website's subpages, should I also add this tag to all of the other subpages of our website? ;
• 
  Federico Sasso: It's supposed to be applied to all pages having an equivalent version in other language, so yes, if your site does.
  Of course, it's an optional tag