Hello Gestumblindi, and thank you for your message!
We use Modèle:Bienvenue nouveau to welcome every new account created on French Wikipedia. The template is distributed by a bot, but it can also be manually distributed of needed. The message is signed by someone from a list, and that person is the point of contact on the wiki for the newcomer. They are not responsible for their edits, though (welcomers don't review what their assigned newcomers do).
We also have more welcome messages, mostly for IPs to encourage them to create an account.
Concerning the mentorship program, I meant by "opt-in" that newcomers had to go there and find someone, contact that person and more or less commit to edit. I guess that's the way it works on German Wikipedia?
We have discovered that this mentorship system is not efficient. The signed welcome message, with a clear call to action ("please contact me if you have any question") is what people are looking for. If they want to edit an article about sports, they don't need to find someone who is interested about sport to know how to ad a citation or use a template. If people have specific questions about sport, they can be redirected to the matching wikiproject. Wikiprojects can also contact newcomers that are working on their field.
This is why the mentorship program is not used much anymore and has been progressively removed from the welcome message. And now, new mentorship relations are based on the welcome message.
I hope that I've replied to your questions. If not, please ask! I'll be very happy to provide you more information. :)