Adding translation to a custom module
Thanks to comments and doc , i have moved the whole module in modules/custom/ and for info file: 

name: 'amu hal'
type: module
description: display publications from HAL
core: 8.x
version: 8.x-3.0
package: AMU
project: amu_hal
'interface translation project': amu_hal
'interface translation server pattern': modules/custom/amu_hal/translations/amu_hal-%language.po


Placing the po file at


  modules/custom/mymodule/translations/mymodule-fr.po


now i could update the translation at 


  /fr/admin/reports/translations


and make string translations

Note that the same po file is not found if the module is placed in sites/mysite/module