Drupal 8+ ship custom translations (.po files) with custom modules

Knowing the old logics from Drupal 7, where you could place a "translations" folder into your custom modules, containing the language .po files, the situation is a bit different for Drupal 8+:

You can find the related information (bit hidden) here:

https://api.drupal.org/api/drupal/core%21modules%21locale%21locale.api.php/group/interface_translation_properties/10

TL;DR:

You need to define the path of the custom translation .po files within your module's .info.yml:

'interface translation project': example_module
'interface translation server pattern': modules/custom/example_module/%project-%version.%language.po 

There's also a good article on this topic here: https://medium.com/limoengroen/how-to-deploy-drupal-interface-translations-5653294c4af6