functions.php

<?php

/**
 * Polylang support for Display Posts Shortcode
 *
 * @author Bill Erickson
 * @see https://wordpress.org/support/topic/display-posts-in-a-specific-language/
 *
 * @param array $args, WP Query arguments
 * @param array $atts, Shortcode attributes
 * @return array $args
 *
 */
function be_polylang_support_dps( $args, $atts ) {
  if( !empty( $atts['lang'] ) )
    $args['lang'] = esc_attr( $atts['lang'] );
  return $args;
}
add_filter( 'display_posts_shortcode_args', 'be_polylang_support_dps', 10, 2 );