If the site is static and using just html then you would need to maintain two separate versions of the site. You could use the same assets though on both of these sites. To share the assets you just need to ensure that any css or spry include references pointed to the same pages. How you would adjust these paths will depend on how you sites are organized. If you have them on the same server but in different sub directories you could just adjust the paths relatively by adding in an extra ../ or two to the path. If the sites exist on different servers then you would want to use absolute paths to the resources and include the full http:// like you would with a link.
If your sites are dynamic there are some other options available to you. You can make the parts of the site that will differ dynamic and come from a db where you have multiple versions of the content. You could use the setting of a session variable to determine the language and which one to show. This is a very custom approach and will require custom coding to implement though.