Drupal 8, translation: .po file in custom module is not loaded

I am trying to add a po file to my custom module. I was following the instruction in the documentation: https://api.drupal.org/api/drupal/core!modules!locale!locale.api.php/gro… Here my code: modules/mymodule/mymodule.info.yml: name: mymodule core: 8.x version: 1 type: module package: mymodule description: My module. interface translation project: mymodule interface translation server[…]