Exporting Custom Menu Translation
So what I ended up doing was to create a module that reads from a json file, and adds the translation. (Import controller + drush command)

a) Load up the custom menu items by menu name:

$main_menu_items = \Drupal::entityTypeManager()-&gt;getStorage('menu_link_content')-&gt;loadByProperties(array('menu_name' =&gt; $menu_name));


b) Go through your menu items, foreach of them check if your menu link title matches one in your json/po file/config.

c) Check if you already have a translation on that menu item, then remove the old translation or skip.

if ($menu_item-&gt;hasTranslation('fr')) {


d) Finally add the translation that exists in your file.

  $menu_item_fr = $menu_item-&gt;addTranslation('fr');
  $menu_item_fr-&gt;title = $translation_json[$menu_name][$link_title];
  $menu_item_fr-&gt;description = '';
  $menu_item_fr-&gt;save();


This is essentially what my json looked like:

{"menu_name": {"english_link_title":"french_link_title"}}


Do hope this helps someone else, or someone can think up a better answer.