Wordpress - How can I manually upgrade translations?

quick-and-dirty hint to solve this problem:

1. find the function list_translation_updates() in wp-admin/update-core.php

   $updates = wp_get_translation_updates();
   if ( ! $updates )                  <- locate the if
   ...
   }
   else {                             <- add this
      print_r ( $updates );
   }
   

2. add the else-case and save the file

3. reload the dashboard page and you will get the desired information.

Similar to @wp_quick_and_dirty's solution, but without editing core files. Add this to your functions.php file:

function translation_updates_list() {
    $translation_updates = wp_get_translation_updates();
    if ( empty($translation_updates) ) { return; }

    echo "<h4>Available translations</h4><pre>";
    echo esc_html( print_r($translation_updates, true) );
    echo "</pre>";
}
add_action('core_upgrade_preamble', 'translation_updates_list');

Based on this even fancier solution: https://gist.github.com/swissspidy/e2d1cde667fa4da4db66