Learn docs how to do it.

Also, Florian DANIEL aka Facyla is created some docs on the french. Perhaps, you can find some usefull information in there.

Thank you RvR , In addition, here are the files that Pascal need to edit. It has been a while since i worked at this plugin. From memory I will try to help you to locate where the language files are. Hopefully one of "these days" I will be able to move all language strings to one file for easier maintenance.

First look for this files and make any changes from English to French.

mod\fb_register_form\views\default\page\layouts\fb_custom_index.php

mod\fb_register_form\views\default\resources\index.php

Then,  make a file called  fr.php  inside language file like this.

mod\fb_register_form\languages\fr.php

Next copy all of the contents from the file called en.php into fr.php

mod\fb_register_form\languages\en.php

<?php

/**
 * User details
 */


return array(
   
    'register:password' => "Enter a combination of at least six numbers,letters (like ! and $).",      
    'register:passwordagain' => "Please Re-enter your Password Again",
    'name' => "Display Name",
    'email' => "Email Address",
    'register:email:address' => "You'll use this email to log in and to reset your password",
    'register:username' => "Please enter your New Username: You'll use it to log in",
);

Finally edit your fr.php with french language for your users.

Here is an example how the above English file should be translated:

mod\fb_register_form\languages\fr.php

<?php

/**
 * French
 */

return array(
     
    'register:password' => "Mot de passe",      
    'register:passwordagain' => "Mot de passe (confirmation)",
    'name' => "Nom affiché",
    'email' => "Adresse email",
    'register:email:address' => "Adresse email",
    'register:username' => "Identifiant",
    
);   

I hope this will  help you started in the right direction...