I start already from a subdomain and this installation was only meant to make a trilingual blog. Thus, at the subject of wildcards to get subdomains I didn't do anything. Now I have a site perfectly running with subdomains again. This is not my first multisite installation, for the former one I think I installed WP MU and if I remember well you had to choose somewhere clearly for OR subdomains OR directories, whereas now I just don't know where to set that. Or does my blog need an update maybe?
Thanks for your reply.