Fail in the upgrade of latest version

Ok I first ask my webhost to make a snapshot of our running website before letting you try to test something. I currently tried on local host and finaly succeed in implementing all component but then I lost all specificities and all the content of the Fr language substitutions.
I come back to you as soon as I get the confirmation of the back-up and send you a PM with the credentials Best regards

I do not recommend providing a live copy of your site. I cannot guarantee the review won't break something.

You should provide a clone of your site instead in a subdomain or subfolder. The best way to do this is to typically make a copy of the database, make a copy of the site folders structure into your subdomain or subfolder, adjust configuration.php to point to your copy of the database, then turn off emails globally in Joomla (so the test site doesn't email your users). Things can then be safely debugged without impacting your actual site.

I've got this message on my administration saying that I haven't fixed a PHP temporary directory Is that can explain the problem ?

Le répertoire temporaire PHP est celui que PHP utilise pour stocker un fichier enregistré avant que Joomla puisse y accéder. Bien que l'absence de définition ne soit pas toujours un problème, si vous avez des erreurs concernant la non-détection des fichiers ou manifestes envoyés, définir ce répertoire dans votre fichier php.ini pourrait régler ce problème.

The PHP temporary directory is the one that PHP uses to store a saved file before Joomla can access it. While the lack of definition is not always a problem, if you have errors regarding not detecting sent files or manifests, setting this directory in your php.ini file might fix this problem.

Yes, that absolutely will cause problems. Files are uploaded to PHP tmp before moved to Joomla tmp for install. If there's a permissions problem with PHP tmp or Joomla tmp then things will either not install or only partially install.

So then do you have a clu about the repertory name I should add to the php.ini ? Actually I have very bad knowledge about that matter

Help could be fine
Best regard

Shall I grab the php.ini file before

Contact your host for server related configuration problems. PHP is not setup correctly if it's tmp directory is not accessible or doesn't exist. This is specifically referring to upload_tmp_dir in php.ini.