MultiSite main page navigation help

Hi all,
I’m sure this is a newb question but I looked around and I am confused on how to do this.
So I originally made a site using WordPress and loaded a theme. I needed the site to be in two languages. So I made a multisite using sub-domains. My original site is http://www.example.com. How do I create a page on this main site that gets rid of my current theme and changes it to a navigation page that allows the visitor to select which language they want, then it directs them to the sub site.
I am looking for simple page with a title, then a US flag saying English and another flag for another language?
Is there a theme for this? Or how do I make this page HTML, since I already have a wordpress site on it

Thanks for any and all help