Automatic sync for posts

I’m working on a multilingual wordpress website using the Polylang plug-in on pages and custom post types.

What I’m looking for is a way to have every post synch automatically, without user input. When creating a new post type, a translation would be automatically created and all contents copied

After some research I figured the method above is used when duplicating/synchronizing posts in bulk:

$polylang = PLL();
$sync_model = new PLL_Sync_Post_Model( $polylang );
$sync_model->copy_post( $post_id, $lang, $sync );

$post_id is the source post id.
$lang is the target language.
$sync is false to simply duplicate the post, true to activate the synhronization.

I tried to attach it to to the ‘save_post’ like this:

function polylang_sync($post_id) {
    $polylang = PLL();
    $sync_model = new PLL_Sync_Post_Model( $polylang );
    $sync_model->copy_post( $post_id, $lang, $sync );
}
add_action( 'save_post', 'polylang_sync' );

And I get the following error message:
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'polylang_sync' not found or invalid function name in (...)wp/wp-includes/class-wp-hook.php on line 290

I’m not sure what I’m doing wrong, or what I could change about this to make it work. I tried variations of this to no avail, I may be taking the wrong approach here.

Thanks in advance