Pretty query arg in custom permalink

Hi,
I have defined a permalink structure which looks like this :

add_filter('post_type_link','product_permalink');
$wp_rewrite->add_rewrite_tag("%product%", '([^/]+)', "product=");
add_filter('post_type_link', 'producttitle_permalink', 10, 3);
$wp_rewrite->add_rewrite_tag("%producttitle%", '([^/]+)', 'producttitle=');

$products_structure = "/Products-DB/Reference-%product%/%producttitle%"
add_permastruct('product', $products_structure, false);

It works very well for now.

Now, I want to add a query arg “COUNTRY” to this permalink, so that visitors can see the post content in ENGLISH, DANSK, FRENCH etc…

the simpliest way is to use:
$wp->add_query_var('country');
then to pass it to WordPress with : https://www.mysite.com/Products-DB/Reference-0012/Product_Title_Sample/?country=ENGLISH

I would like to use a pretty permalink like this :
https://www.mysite.com/Products-DB/ENGLISH/Reference-0012/Product_Title_Sample/

How can I do that ? Is add_rewrite_tag the best way to do that ?

Thank you for your help,
Looic.