允许在url中使用可选前缀

I am using wordpress and I have the following URL

http://www.example.com/product/sony-xperia/

What I am trying to achieve is if I add fr after the base url like the example below, I want to display the same content as I can see for the url above.

http://www.example.com/fr/product/sony-xperia/

Could you please tell me how to achieve this?

I have managed to do it. I wrote the following piece of code in my function.php file and it worked.

function custom_rewrite_basic() 
{   
  add_rewrite_rule('^fr/product/([^/]*)/?', 'index.php?product=$matches[1]', 'top');
}
add_action('init', 'custom_rewrite_basic');