Using URL rewrite to add custom URL for custom post type

Question

I'm using ACF to make a multilingual site. My custom post URLs must work both with and without /en/ as prefix. E.g. mysite.com/posttype/any-slug/ and mysite.com/en/posttype/any-slug/ must display the same post.

I've tried using this rewrite rule in functions.php (and some variations of it), but have trouble getting it to work:

add_action('init', 'add_my_rule');

function add_my_rule() {
    add_rewrite_rule('^en\/posttype\/[.+]$','index.php?pagename=$matches[1]','top');
}

I've noticed there's other questions with similar titles, but the cases seem to be different from this one.

Answer 1

pagename assumes the page post type. You need to use the custom post type query var in your rule instead. Presumably you also want to detect that this is a request for en, you can add your own rewrite tag to store that, then add that to the rule:

add_action('init', 'add_my_rule');

function add_my_rule() {
    add_rewrite_tag('%my_language%', '([^&]+)');
    add_rewrite_rule(
        '^en/posttype/([^/]*)?/?$',
        'index.php?posttype=$matches[1]&my_language=en',
        'top'
    );
}