wpml_object_id

// will return the post ID in the current language for post ID 1
echo apply_filters( 'wpml_object_id', 1, 'post' );

// will return the category ID in the current language for categoy ID 4. If the translation is missing it will return the original (here: category ID 4)
echo apply_filters( 'wpml_object_id', 4, 'category', TRUE  );

// will return the German attachment ID for attachment ID 25. If the translation is missing it will return NULL
echo apply_filters( 'wpml_object_id', 25, 'attachment', FALSE, 'de' );

/**
 * Returns the translated object ID(post_type or term) or original if missing
 *
 * @param $object_id integer|string|array The ID/s of the objects to check and return
 * @param $type the object type: post, page, {custom post type name}, nav_menu, nav_menu_item, category, tag etc.
 * @return string or array of object ids
 */
function my_translate_object_id( $object_id, $type ) {
    $current_language= apply_filters( 'wpml_current_language', NULL );
    // if array
    if( is_array( $object_id ) ){
        $translated_object_ids = array();
        foreach ( $object_id as $id ) {
            $translated_object_ids[] = apply_filters( 'wpml_object_id', $id, $type, true, $current_language );
        }
        return $translated_object_ids;
    }
    // if string
    elseif( is_string( $object_id ) ) {
        // check if we have a comma separated ID string
        $is_comma_separated = strpos( $object_id,"," );

        if( $is_comma_separated !== FALSE ) {
            // explode the comma to create an array of IDs
            $object_id     = explode( ',', $object_id );

            $translated_object_ids = array();
            foreach ( $object_id as $id ) {
                $translated_object_ids[] = apply_filters ( 'wpml_object_id', $id, $type, true, $current_language );
            }

            // make sure the output is a comma separated string (the same way it came in!)
            return implode ( ',', $translated_object_ids );
        }
        // if we don't find a comma in the string then this is a single ID
        else {
            return apply_filters( 'wpml_object_id', intval( $object_id ), $type, true, $current_language );
        }
    }
    // if int
    else {
        return apply_filters( 'wpml_object_id', $object_id, $type, true, $current_language );
    }
}