Make assigning post to a specific category equivalent to assigning it to all categories

One way to deal with the problem is to use the redirect_canonical filter to cancel the redirect (that happens when the category that the post is assigned to doesn’t match the category slug in the requested post URL) for posts assigned to International (intl) category.

/*
 * Related core file: wp-includes/canonical.php
 *
 * NOTE: If you see no change after adding the function, try re-saving the permalink settings.
 */

add_filter( 'redirect_canonical', 'itsme_redirect_canonical', 10, 2 );
function itsme_redirect_canonical( $redirect_url, $requested_url ) {
    global $wp_rewrite;

    if( is_single() && in_category( 'intl' ) && strpos( $wp_rewrite->permalink_structure, '%category%' ) !== false && $cat = get_query_var( 'category_name' ) ) {

        $category = get_category_by_path( $cat );

        if( $category && !is_wp_error( $category ) ) {

            return str_replace( 'intl', $cat, $redirect_url );

            // return $requested_url;

            /*
             * WHY NOT SIMPLY `return $requested_url;`?
             *
             * By returning $requested_url you're returning something that might not be
             * canonical (e.g. extra slashes, etc) and there's a possibility that the
             * safety rules that WordPress has in place will redirect your URL back to
             * the (original) canonical URL.
             *
             * For example, this will work fine:
             *
             *  http://example.com/in/2013/11/02/sample-post/
             *
             * But this:
             *
             *  http://example.com/in/2013/11/02/sample-post/////
             *
             * Will redirect back to the original, canonical URL, i.e.,
             *
             *  http://example.com/intl/2013/11/02/sample-post/
             *
             * In order to avoid that, you'll additionally have to write some safety
             * rules so that a *corrected* URL, rather than simply the requested one, is
             * returned, in which case, you may need to use `str_replace` or `preg_replace`
             * to remove the extra characters, among other rules needed, if any. Now that
             * can get complex.
             *
             * So instead of dealing with all this mess ourselves, we are allowing
             * WordPress to redirect to the (original) canonical URL, and at that point,
             * we are replacing 'intl' (which is the category slug in the canonical URL)
             * with the category slug in the requested URL.
             */

        }

    }

    return $redirect_url;
}

The function may not be perfect, in which case any corrections are welcome!

Thanks to @Zogot for the idea, and @StephenHarris for the idea and all the help.