Use custom template on certain URLs

Thanks to @Milo on the comments, I’m gonna post my answer:

Firstly I added this rule:

add_rewrite_rule(  
    "en/artist/([^/]+)/?",  
    'index.php?pagename=post_type=artist&artist=$matches[1]&gal_template=en',
    "top"); 

Then this filter:

add_filter( 'query_vars', 'gal_query_vars' );
function gal_query_vars( $query_vars )
{
    $query_vars[] = 'gal_template';
    return $query_vars;
}

Finally, I filtered the single template:

add_filter( 'single_template', 'get_custom_post_type_template' );
function get_custom_post_type_template($single_template) {
     global $post;

     if ($post->post_type == 'artist') {

            $q = get_query_var('gal_template');

            if ($q === 'en') {
                $single_template = dirname( __FILE__ ) . '/single-artist-en.php';
            }


     }
     return $single_template;
}

Then on single-artist-en.php I did:

get_template_part('single-artist')

But you could do whatever you need.