Prefill new Polylang translations with the Etch layout

<?php
/**
 * Etch + Polylang Free
 *
 * Prefill a new translation opened with Polylang's "+" link using the
 * source post's stored Etch blocks. WordPress handles saving and sanitization.
 * This does not clone component definitions, metadata, or linked documents.
 */
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

function ed_etch_pll_translation_source( $new_post ) {
	global $pagenow;

	if (
		! is_admin()
		|| 'post-new.php' !== $pagenow
		|| ! ( $new_post instanceof WP_Post )
		|| 'auto-draft' !== $new_post->post_status
		|| $new_post->ID <= 0
		|| ! isset(
			$_GET['from_post'],
			$_GET['new_lang'],
			$_GET['post_type'],
			$_GET['_wpnonce']
		)
	) {
		return null;
	}

	foreach ( array( 'from_post', 'new_lang', 'post_type', '_wpnonce' ) as $parameter ) {
		if ( ! is_string( $_GET[ $parameter ] ) ) {
			return null;
		}
	}

	// Reject malformed IDs instead of coercing them into another post's ID.
	$source_id = filter_var(
		wp_unslash( $_GET['from_post'] ),
		FILTER_VALIDATE_INT,
		array( 'options' => array( 'min_range' => 1 ) )
	);
	$new_lang  = wp_unslash( $_GET['new_lang'] );
	$post_type = wp_unslash( $_GET['post_type'] );
	$nonce     = wp_unslash( $_GET['_wpnonce'] );

	if (
		! $source_id
		|| $source_id === (int) $new_post->ID
		|| '' === $new_lang
		|| sanitize_key( $new_lang ) !== $new_lang
		|| sanitize_key( $post_type ) !== $post_type
		|| ! wp_verify_nonce( $nonce, 'new-post-translation' )
		|| $post_type !== $new_post->post_type
		|| ! function_exists( 'pll_is_translated_post_type' )
		|| ! function_exists( 'pll_languages_list' )
		|| ! function_exists( 'pll_get_post_language' )
		|| ! function_exists( 'pll_get_post' )
		|| ! pll_is_translated_post_type( $post_type )
	) {
		return null;
	}

	$languages = pll_languages_list( array( 'fields' => 'slug' ) );

	if ( ! is_array( $languages ) || ! in_array( $new_lang, $languages, true ) ) {
		return null;
	}

	$source = get_post( $source_id );

	if (
		! ( $source instanceof WP_Post )
		|| $source->post_type !== $post_type
		|| in_array( $source->post_status, array( 'trash', 'auto-draft' ), true )
		|| ! current_user_can( 'edit_post', $source_id )
		|| ! current_user_can( 'edit_post', $new_post->ID )
		|| false === strpos( (string) $source->post_content, '<!-- wp:etch/' )
	) {
		return null;
	}

	$post_type_object = get_post_type_object( $post_type );

	if ( ! $post_type_object ) {
		return null;
	}

	$create_capability = isset( $post_type_object->cap->create_posts )
		? $post_type_object->cap->create_posts
		: $post_type_object->cap->edit_posts;

	if ( ! current_user_can( $create_capability ) ) {
		return null;
	}

	$source_lang = pll_get_post_language( $source_id, 'slug' );

	if ( ! is_string( $source_lang ) || '' === $source_lang || $new_lang === $source_lang ) {
		return null;
	}

	// Do not prefill a duplicate translation. The current auto-draft is allowed.
	$existing_id = (int) pll_get_post( $source_id, $new_lang );

	if ( $existing_id && $existing_id !== (int) $new_post->ID ) {
		return null;
	}

	return $source;
}

function ed_etch_pll_default_content( $content, $new_post ) {
	if ( ! is_string( $content ) || '' !== trim( $content ) ) {
		return $content;
	}

	$source = ed_etch_pll_translation_source( $new_post );

	return $source
		? (string) $source->post_content
		: $content;
}
add_filter( 'default_content', 'ed_etch_pll_default_content', 20, 2 );

/*
 * Prefill the title and excerpt only when no earlier filter supplied a value.
 * Remove either filter registration below to leave that field empty.
 */
function ed_etch_pll_default_title( $title, $new_post ) {
	if ( ! is_string( $title ) || '' !== trim( $title ) ) {
		return $title;
	}

	$source = ed_etch_pll_translation_source( $new_post );

	return $source
		? (string) $source->post_title
		: $title;
}
add_filter( 'default_title', 'ed_etch_pll_default_title', 20, 2 );

function ed_etch_pll_default_excerpt( $excerpt, $new_post ) {
	if ( ! is_string( $excerpt ) || '' !== trim( $excerpt ) ) {
		return $excerpt;
	}

	$source = ed_etch_pll_translation_source( $new_post );

	return $source
		? (string) $source->post_excerpt
		: $excerpt;
}
add_filter( 'default_excerpt', 'ed_etch_pll_default_excerpt', 20, 2 );