The details they provide do look pretty confusing.
It’s worth trying the database server IP address they provide in place of ‘localhost’ in define(‘DB_HOST’, ‘localhost’);. The database may be on a different server to your site files, in which case ‘localhost’ won’t work.
I’d try 35.21.239.28 followed by 11.17.26.5
I don’t speak French but I think you’re facing a permissions issue.
Open terminal in Ubuntu (ctrl alt+t) and enter:
sudo chown -R www-data /var/www/html/yoursite
This recursively gives ownership to your site files to www-data (the apache group). You should then be able to import themes/plugins etc through the WordPress dashboard.
Once you have done this, if you need to change ownership back to your user (maybe you are editing theme files) enter:
sudo chown -R $USER /var/www/html/yoursite
Good luck
