Advanced custom fields: make option pages with a custom post_id translatable.

functions.php

<?php

add_filter('acf/pre_load_post_id',
    /**
     * Fix all ACF option pages. acf-pro translates the default "options" page, but not option pages with a custom post_id
     *
     * @param string|null $return
     * @param string|int  $post_id
     *
     * @return null|string
     * @see acf_get_valid_post_id
     * @see acf_add_options_page
     * @see acf_add_options_sub_page
     */
    static function ($return, $post_id) {
        if ($return !== null) {
            return $return;
        }

        $current_language_iso_code = acf_get_setting('current_language');
        $default_language_iso_code = acf_get_setting('default_language');
        if ($current_language_iso_code && $current_language_iso_code !== $default_language_iso_code) {

            $pages = acf_get_options_pages();
            if (!is_array($pages)) {
                $pages = [];
            }
            $option_pages_post_ids = array_map('strval', array_column($pages, 'post_id'));

            if (in_array((string)$post_id, $option_pages_post_ids, true)) {

                $return = $post_id . '_' . $current_language_iso_code;

                //var_dump('acf/pre_load_post_id', compact('post_id', 'default_language_iso_code', 'current_language_iso_code', 'return'));
            }
        }
        return $return;
    }, 10, 2);