Translation server suggests translation with old date 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?
i didn't find answer for none of my questions but at least find a solution:

after testing clear cache without luck i did these steps:

user interface translation->setting->change translation source to 'local file only'->check update translation manually

and old translation suggestion was disappeared.

now with changing translation source to Drupal 'translation server and local files' i get related translation suggestion for my Drupal version.