agaric-coop/web/themes/custom/agarica/agarica.theme

<?php

use \Drupal\block\Entity\Block;

/**
 * Implements hook_preprocess_region__footer().
 */
function agarica_preprocess_region__footer(&$variables) {
  $block = Block::load('languageswitcher');
  $block_content = \Drupal::entityTypeManager()
    ->getViewBuilder('block')
    ->view($block);
  $variables['language_switcher'] = $block_content;
}