Translation and display problems

I’m sorry but with regards to the translation questions, I’m not really sure what you’re asking.

The French translation is 93% complete, so there are some untranslated strings. As those are translated in the project, the translation files will be updated:
https://translate.wordpress.org/projects/wp-plugins/wp-members

Be sure to utilize the translation from the link above (which should load automatically to your /wp-content/languages/plugins/ folder). There is a French translation that is still included in the plugin’s download, but that is a legacy file and should not be used.

For your question about editing strings in your admin panel, there is a premium extension for the plugin that will do that. Note, however, that it is only for the user-facing text the plugin displays (not anything in the admin).
https://rocketgeek.com/plugins/wp-members/extensions/editor/

Second, when I want to customize my page and I go to “customize” => “Wp-Members”. Then. I choose the page I want to customize and I check the “display form as not connected” button. Anyway, it doesn’t work. I can’t see my page as not connected.

Just to be clear, this is only a feature for the plugin’s forms and is specific to the forms generated by the plugin’s shortcodes. So it only displays the logged-out view of the login and registration forms when viewing a page that has either the [wpmem_form login] or [wpmem_form register]. It’s purpose is really just for being able to work on custom CSS in the WP Customizer. There is more information on this in the plugin’s documentation: https://rocketgeek.com/plugins/wp-members/docs/customizing-forms/using-the-wordpress-customizer/