Discussion:
Some pages are only accessible to registered members.
What are you trying to achieve:
The site I manage is written in French, so I personalised the signUp and login forms. it works well.
But when a new member signsup, he receives another standard confirmation window in English.
How can I add this other window (and others if any) in the personalised part of my pages?
Hi sentiers5140
Have you looked in the language options in your Site’s dashboard? There you should be able to change it to French, which then should adjust all Modals/Popups.
Hope that helps 
Site Dashboard is as followed:
Site langage : french(France)
Regional format: Français(Belgique)
Account langage : Français
so seems correct for me.
For site langage, the information says it affects the langage of some installed applications.
It seems it is not the case for member messages, or I missed something