Moved to Support. Please be sure to post in the correct section.
I would just create two separate posts, one Spanish, one English. Then edit each one to add a link to the other. You can use the html editor to enclose each link in a <div> with button styling. You can copy the default button style that Graphene uses, if you like, like this:
<div class="block-button">link stuff goes here</div>
This has also been discussed previously in this thread:
https://forum.graphene-theme.com/graphene-support/how-to-create-a-site-version-on-second-language
I’m not sure that we came up with anything really useful, but click on Tomy1734’s site to see how he arranged things. You might prefer that method. But I rather like the idea of a button.