Create your module with mymodule.info.yml
name: 'My Module'
description: 'Custom Module'
package: Custom
type: module
core: 8.x
and then add this code to .info.yml file to define your translation file:
project: mymodule
'interface translation project': mymodule
'interface translation server pattern': modules/custom/mymodule/translations/mymodule-%language.po
Placing the po file at modules/custom/mymodule/translations/mymodule-fr.po for example for french language.
now you could update the translation at /fr/admin/reports/translations
and dont forget to clear the cache :)
