Link to another theme

Hi all, I’m kinda new to WP and I want to make a two different languages in my site(not WP admin menu, it will be the same language). I want it to be two different versions of site(by design) and of course a link in each of this themes to another theme, so it will be possible in any moment to switch a theme from one to two or from two to one.
What is the code I should use to achieve this kinda effect?
Thanks!