Spanish/english site, but not entirely multilingual

Hello everyone! I´m trying to set a dual language site. The original is in spanish, and we want few pages in english too, with a switch botton on header. 

How is the simplest way to achieve this? (I already have all the pages manually translated that we want). 

Thanks a lot!

My ideal is add all pages to header, then use CSS code to hide some items on specific page

I wrote a quick guide on this, you can see #1 code