Polylang: Copy ACF fields from default language

pll_copy_post_metas_from_default_lang.php

/*
 * Copy meta fields across languages
 */

$acf_keys_to_copy = [
	'acf_singular_field_name',
	'acf_repeater_field_name' => ['acf_repeater_subfield_name']
];

add_filter( 'pll_copy_post_metas', function( $keys, $sync, $from, $to, $lang ) use ( $acf_keys_to_copy ) {

	if ( pll_get_post_language( $from ) === pll_default_language() ) {
		$keys = array_merge( $keys, get_copyable_meta_keys( $from, $to, $acf_keys_to_copy ) );
	}

	return $keys;

}, 10, 5 );

function get_copyable_meta_keys( $from, $to, $acf_keys_to_copy ) {

	$keys_to_copy = [];

	// Get custom meta keys from source post. Also get keys from destination post,
	// allowing to synchronize deleted fields.
	$all_post_meta_keys = array_unique( array_merge(
		array_keys( get_post_custom( $from ) ),
		array_keys( get_post_custom( $to ) )
	) );

	// Divide singular/repeater fields into separate arrays
	foreach ( $acf_keys_to_copy as $key => $value ) {
		if ( is_array( $value ) ) {
			$acf_keys_to_copy_repeater[$key] = $value;
			$acf_keys_to_copy_singular[] = $key;
		} else {
			$acf_keys_to_copy_singular[] = $value;
		}
	}

	// Loop through all post meta keys to determine which whould be copied
	foreach ( $all_post_meta_keys as $post_meta_key ) {

		if ( in_array( $post_meta_key, $acf_keys_to_copy_singular ) ) {

			// Normal, singular keys
			$keys_to_copy[] = $post_meta_key;

		} else {

			// Repeater fields.
			// Check if the post meta key matches the key structure of a syncable ACF repeater -field.

			foreach ( $acf_keys_to_copy_repeater as $acf_field_key => $acf_subfield_keys ) {

				$post_meta_key_matches_acf_repeater = false;

				foreach ( $acf_subfield_keys as $acf_subfield_key ) {
					if ( preg_match( "#{$acf_field_key}_[0-9+]_{$acf_subfield_key}$#", $post_meta_key ) ) {
						$post_meta_key_matches_acf_repeater = true;
						break;
					}
				}

				if ( $post_meta_key_matches_acf_repeater ) {
					$keys_to_copy[] = $post_meta_key;
					break;
				}

			}

		}

	}

	return $keys_to_copy;

}

// Make copyable fields read-only for translations
add_action('admin_head', function() use ( $acf_keys_to_copy ) {

	global $post;

	if ( get_current_screen()->id === 'my_custom_post_type' && pll_get_post_language( $post->ID ) !== pll_default_language() ) {

		$field_keys = [];

		// Get field keys
		foreach ( $acf_keys_to_copy as $key => $value ) {
			if ( is_array( $value ) ) {
				$field_keys[] = $key;
			} else {
				$field_keys[] = $value;
			}
		}

		$css = '<style>';

		foreach ( $field_keys as $i => $key ) {
			$css .= '.acf-field[data-name="' . $key . '"]' . ( next( $field_keys ) ? ',' : '' );
		}

		$css .= ' { pointer-events: none; opacity: 0.3; -webkit-user-select: none; user-select: none; cursor: not-allowed; }';
		$css .= '</style>';

		echo $css;

	}

} );

Thank you so much for writing this. This is exactly what I needed + it prevent editing the acf in the secondary language. This is really really nice. You really made my day !