Drupal 8 implementation of hook_language_switch_links_alter that replaces links in language switcher block with links to front page.

d8-language-switcher-to-front.php

<?php

use Drupal\Core\Language\LanguageInterface;
use Drupal\Core\Url;

/**
 * Implements hook_language_switch_links_alter().
 *
 * Replaces links in language switcher block with links to front page.
 */
function MYMODULE_language_switch_links_alter(array &$links, $type, $path) {

  if ($type == LanguageInterface::TYPE_INTERFACE) {
    $front = new Url('<front>');
    foreach ($links as &$link) {
      $link['url'] = $front;
    }
  }
}