in admin/config/regional/translate/settings page, i had Check for updates set to never and translation server to Drupal translation server and local files.
i recently updated my site drupal version to 8.6.17.
after check for translation update manually i get a update available for drupal core translation with 2018 date for one of languages and i can't find .po file with this creation date.
my questions are:
how drupal check for available translations?
is it possible that it suggests translation for a older drupal version?
and most important is it safe to delete .po files from translation folder on my site manually and run check again?