As discussed over IRC, I was under the impression that subject lines for the Thank you emails gets deployed automatically. Elliott confirmed that that is not the case. Deploying the subject line for the Thank you email in French should, according to tech, not be a complicated task. If this deployment could happen shortly, that would be fantastic. :)
https://translatewiki.net/wiki/MediaWiki:Donate_interface-email-subject/fr
Thank you.