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 pattern: modules/mymodule/translations/%language.po

modules/mymodule/translations/fr.po:

msgid "hello"
msgstr "bonjour"

I tried to clear cache, and uninstall/install my module. But the translation does not seems to get loaded.
Any idea of what could be missing ?

Drupal version: 


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